Add dark mode settings

Closes #5394
See #2377

6 files changed, 534 insertions, 2 deletions

diff --git a/doc/changelog.asciidoc b/doc/changelog.asciidoc
index d6e42e2b6..a6aead789 100644
--- a/doc/changelog.asciidoc
+++ b/doc/changelog.asciidoc
@@ -41,6 +41,9 @@ Added
 - New `colors.contextmenu.disabled.{fg,bg}` settings to customize colors for
   disabled items in the context menu.
 - New line selection mode (`:toggle-selection --line`), bound to `Shift-V` in caret mode.
+- New `colors.webpage.darkmode.*` settings to control Chromium's dark mode.
+  Note that those settings only work with QtWebEngine on Qt >= 5.14 and require
+  a restart of qutebrowser.
 
 Changed
 ~~~~~~~
diff --git a/doc/help/settings.asciidoc b/doc/help/settings.asciidoc
index 4c20126c3..3787238a9 100644
--- a/doc/help/settings.asciidoc
+++ b/doc/help/settings.asciidoc
@@ -111,6 +111,15 @@
 |<<colors.tabs.selected.odd.bg,colors.tabs.selected.odd.bg>>|Background color of selected odd tabs.
 |<<colors.tabs.selected.odd.fg,colors.tabs.selected.odd.fg>>|Foreground color of selected odd tabs.
 |<<colors.webpage.bg,colors.webpage.bg>>|Background color for webpages if unset (or empty to use the theme's color).
+|<<colors.webpage.darkmode.algorithm,colors.webpage.darkmode.algorithm>>|Which algorithm to use for modifying how colors are rendered with darkmode.
+|<<colors.webpage.darkmode.contrast,colors.webpage.darkmode.contrast>>|Contrast for dark mode.
+|<<colors.webpage.darkmode.enabled,colors.webpage.darkmode.enabled>>|Render all web contents using a dark theme.
+|<<colors.webpage.darkmode.grayscale.all,colors.webpage.darkmode.grayscale.all>>|Render all colors as grayscale.
+|<<colors.webpage.darkmode.grayscale.images,colors.webpage.darkmode.grayscale.images>>|Desaturation factor for images in dark mode.
+|<<colors.webpage.darkmode.policy.images,colors.webpage.darkmode.policy.images>>|Which images to apply dark mode to.
+|<<colors.webpage.darkmode.policy.page,colors.webpage.darkmode.policy.page>>|Which pages to apply dark mode to.
+|<<colors.webpage.darkmode.threshold.background,colors.webpage.darkmode.threshold.background>>|Threshold for inverting background elements with dark mode.
+|<<colors.webpage.darkmode.threshold.text,colors.webpage.darkmode.threshold.text>>|Threshold for inverting text with dark mode.
 |<<colors.webpage.prefers_color_scheme_dark,colors.webpage.prefers_color_scheme_dark>>|Force `prefers-color-scheme: dark` colors for websites.
 |<<completion.cmd_history_max_items,completion.cmd_history_max_items>>|Number of commands to save in the command history.
 |<<completion.delay,completion.delay>>|Delay (in milliseconds) before updating completions after typing a character.
@@ -1534,6 +1543,160 @@ Type: <<types,QtColor>>
 
 Default: +pass:[white]+
 
+[[colors.webpage.darkmode.algorithm]]
+=== colors.webpage.darkmode.algorithm
+Which algorithm to use for modifying how colors are rendered with darkmode.
+This setting requires a restart.
+
+Type: <<types,String>>
+
+Valid values:
+
+ * +lightness-cielab+: Modify colors by converting them to CIELAB color space and inverting the L value.
+ * +lightness-hsl+: Modify colors by converting them to the HSL color space and inverting the lightness (i.e. the "L" in HSL).
+ * +brightness-rgb+: Modify colors by subtracting each of r, g, and b from their maximum value.
+
+Default: +pass:[lightness-cielab]+
+
+On QtWebEngine, this setting requires Qt 5.14 or newer.
+
+On QtWebKit, this setting is unavailable.
+
+[[colors.webpage.darkmode.contrast]]
+=== colors.webpage.darkmode.contrast
+Contrast for dark mode.
+This only has an effect when `colors.webpage.darkmode.algorithm` is set to `lightness-hsl` or `brightness-rgb`.
+This setting requires a restart.
+
+Type: <<types,Float>>
+
+Default: +pass:[0.0]+
+
+On QtWebEngine, this setting requires Qt 5.14 or newer.
+
+On QtWebKit, this setting is unavailable.
+
+[[colors.webpage.darkmode.enabled]]
+=== colors.webpage.darkmode.enabled
+Render all web contents using a dark theme.
+Example configurations from Chromium's `chrome://flags`:
+
+- "With simple HSL/CIELAB/RGB-based inversion": Set
+  `colors.webpage.darkmode.algorithm` accordingly.
+
+- "With selective image inversion": Set
+  `colors.webpage.darkmode.policy.images` to `smart`.
+
+- "With selective inversion of non-image elements": Set
+  `colors.webpage.darkmode.threshold.text` to 150 and
+  `colors.webpage.darkmode.threshold.background` to 205.
+
+- "With selective inversion of everything": Combines the two variants
+  above.
+This setting requires a restart.
+
+Type: <<types,Bool>>
+
+Default: +pass:[false]+
+
+On QtWebEngine, this setting requires Qt 5.14 or newer.
+
+On QtWebKit, this setting is unavailable.
+
+[[colors.webpage.darkmode.grayscale.all]]
+=== colors.webpage.darkmode.grayscale.all
+Render all colors as grayscale.
+This only has an effect when `colors.webpage.darkmode.algorithm` is set to `lightness-hsl` or `brightness-rgb`.
+This setting requires a restart.
+
+Type: <<types,Bool>>
+
+Default: +pass:[false]+
+
+On QtWebEngine, this setting requires Qt 5.14 or newer.
+
+On QtWebKit, this setting is unavailable.
+
+[[colors.webpage.darkmode.grayscale.images]]
+=== colors.webpage.darkmode.grayscale.images
+Desaturation factor for images in dark mode.
+If set to 0, images are left as-is. If set to 1, images are completely grayscale. Values between 0 and 1 desaturate the colors accordingly.
+This setting requires a restart.
+
+Type: <<types,Float>>
+
+Default: +pass:[0.0]+
+
+On QtWebEngine, this setting requires Qt 5.14 or newer.
+
+On QtWebKit, this setting is unavailable.
+
+[[colors.webpage.darkmode.policy.images]]
+=== colors.webpage.darkmode.policy.images
+Which images to apply dark mode to.
+This setting requires a restart.
+
+Type: <<types,String>>
+
+Valid values:
+
+ * +always+: Apply dark mode filter to all images.
+ * +never+: Never apply dark mode filter to any images.
+ * +smart+: Apply dark mode based on image content.
+
+Default: +pass:[never]+
+
+On QtWebEngine, this setting requires Qt 5.14 or newer.
+
+On QtWebKit, this setting is unavailable.
+
+[[colors.webpage.darkmode.policy.page]]
+=== colors.webpage.darkmode.policy.page
+Which pages to apply dark mode to.
+This setting requires a restart.
+
+Type: <<types,String>>
+
+Valid values:
+
+ * +always+: Apply dark mode filter to all frames, regardless of content.
+ * +smart+: Apply dark mode filter to frames based on background color.
+
+Default: +pass:[smart]+
+
+On QtWebEngine, this setting requires Qt 5.14 or newer.
+
+On QtWebKit, this setting is unavailable.
+
+[[colors.webpage.darkmode.threshold.background]]
+=== colors.webpage.darkmode.threshold.background
+Threshold for inverting background elements with dark mode.
+Background elements with brightness above this threshold will be inverted, and below it will be left as in the original, non-dark-mode page.  Set to 256 to never invert the color or to 0 to always invert it.
+Note: This behavior is the opposite of `colors.webpage.darkmode.threshold.text`!
+This setting requires a restart.
+
+Type: <<types,Int>>
+
+Default: +pass:[0]+
+
+On QtWebEngine, this setting requires Qt 5.14 or newer.
+
+On QtWebKit, this setting is unavailable.
+
+[[colors.webpage.darkmode.threshold.text]]
+=== colors.webpage.darkmode.threshold.text
+Threshold for inverting text with dark mode.
+Text colors with brightness below this threshold will be inverted, and above it will be left as in the original, non-dark-mode page.  Set to 256 to always invert text color or to 0 to never invert text color.
+This setting requires a restart.
+
+Type: <<types,Int>>
+
+Default: +pass:[256]+
+
+On QtWebEngine, this setting requires Qt 5.14 or newer.
+
+On QtWebKit, this setting is unavailable.
+
 [[colors.webpage.prefers_color_scheme_dark]]
 === colors.webpage.prefers_color_scheme_dark
 Force `prefers-color-scheme: dark` colors for websites.
diff --git a/qutebrowser/config/configdata.yml b/qutebrowser/config/configdata.yml
index 46b745089..dc49e4cdd 100644
--- a/qutebrowser/config/configdata.yml
+++ b/qutebrowser/config/configdata.yml
@@ -2612,6 +2612,169 @@ colors.webpage.prefers_color_scheme_dark:
     QtWebEngine: Qt 5.14
     QtWebKit: false
 
+## dark mode
+
+# darkModeClassifierType is not exposed, as the icon classifier isn't actually
+# implemented in Chromium:
+#
+# https://source.chromium.org/chromium/chromium/src/+/master:third_party/blink/renderer/platform/graphics/dark_mode_icon_classifier.cc
+
+colors.webpage.darkmode.enabled:
+  default: false
+  type: Bool
+  desc: >-
+    Render all web contents using a dark theme.
+
+    Example configurations from Chromium's `chrome://flags`:
+
+
+    - "With simple HSL/CIELAB/RGB-based inversion": Set
+      `colors.webpage.darkmode.algorithm` accordingly.
+
+    - "With selective image inversion": Set
+      `colors.webpage.darkmode.policy.images` to `smart`.
+
+    - "With selective inversion of non-image elements": Set
+      `colors.webpage.darkmode.threshold.text` to 150 and
+      `colors.webpage.darkmode.threshold.background` to 205.
+
+    - "With selective inversion of everything": Combines the two variants
+      above.
+  restart: true
+  backend:
+    QtWebEngine: Qt 5.14
+    QtWebKit: false
+
+colors.webpage.darkmode.algorithm:
+  default: lightness-cielab
+  desc: "Which algorithm to use for modifying how colors are rendered with
+    darkmode."
+  type:
+    name: String
+    valid_values:
+      - lightness-cielab: Modify colors by converting them to CIELAB color
+            space and inverting the L value.
+      - lightness-hsl: Modify colors by converting them to the HSL color space
+            and inverting the lightness (i.e. the "L" in HSL).
+      - brightness-rgb: Modify colors by subtracting each of r, g, and b from
+            their maximum value.
+      # kSimpleInvertForTesting is not exposed, as it's equivalent to
+      # kInvertBrightness without gamma correction, and only available for
+      # Chromium's automated tests
+  restart: true
+  backend:
+    QtWebEngine: Qt 5.14
+    QtWebKit: false
+
+colors.webpage.darkmode.contrast:
+  default: 0.0
+  type:
+    name: Float
+    minval: -1.0
+    maxval: 1.0
+  desc: >-
+    Contrast for dark mode.
+
+    This only has an effect when `colors.webpage.darkmode.algorithm` is set to
+    `lightness-hsl` or `brightness-rgb`.
+  restart: true
+  backend:
+    QtWebEngine: Qt 5.14
+    QtWebKit: false
+
+colors.webpage.darkmode.policy.images:
+  default: never
+  type:
+    name: String
+    valid_values:
+      - always: Apply dark mode filter to all images.
+      - never: Never apply dark mode filter to any images.
+      - smart: Apply dark mode based on image content.
+  desc: Which images to apply dark mode to.
+  restart: true
+  backend:
+    QtWebEngine: Qt 5.14
+    QtWebKit: false
+
+colors.webpage.darkmode.policy.page:
+  default: smart
+  type:
+    name: String
+    valid_values:
+      - always: Apply dark mode filter to all frames, regardless of content.
+      - smart: Apply dark mode filter to frames based on background color.
+  desc: Which pages to apply dark mode to.
+  restart: true
+  backend:
+    QtWebEngine: Qt 5.14
+    QtWebKit: false
+
+colors.webpage.darkmode.threshold.text:
+  default: 256
+  type:
+    name: Int
+    minval: 0
+    maxval: 256
+  desc: >-
+      Threshold for inverting text with dark mode.
+
+      Text colors with brightness below this threshold will be inverted, and
+      above it will be left as in the original, non-dark-mode page.  Set to 256
+      to always invert text color or to 0 to never invert text color.
+  restart: true
+  backend:
+    QtWebEngine: Qt 5.14
+    QtWebKit: false
+
+colors.webpage.darkmode.threshold.background:
+  default: 0
+  type:
+    name: Int
+    minval: 0
+    maxval: 256
+  desc: >-
+      Threshold for inverting background elements with dark mode.
+
+      Background elements with brightness above this threshold will be inverted,
+      and below it will be left as in the original, non-dark-mode page.  Set to
+      256 to never invert the color or to 0 to always invert it.
+
+      Note: This behavior is the opposite of
+      `colors.webpage.darkmode.threshold.text`!
+  restart: true
+  backend:
+    QtWebEngine: Qt 5.14
+    QtWebKit: false
+
+colors.webpage.darkmode.grayscale.all:
+  default: false
+  type: Bool
+  desc: >-
+    Render all colors as grayscale.
+
+    This only has an effect when `colors.webpage.darkmode.algorithm` is set to
+    `lightness-hsl` or `brightness-rgb`.
+  restart: true
+  backend:
+    QtWebEngine: Qt 5.14
+    QtWebKit: false
+
+colors.webpage.darkmode.grayscale.images:
+  default: 0.0
+  type:
+    name: Float
+    minval: 0.0
+    maxval: 1.0
+  desc: >-
+    Desaturation factor for images in dark mode.
+
+    If set to 0, images are left as-is. If set to 1, images are completely
+    grayscale. Values between 0 and 1 desaturate the colors accordingly.
+  restart: true
+  backend:
+    QtWebEngine: Qt 5.14
+    QtWebKit: false
+
 # emacs: '
 
 ## fonts
diff --git a/qutebrowser/config/configinit.py b/qutebrowser/config/configinit.py
index b15225210..3c80cfe1b 100644
--- a/qutebrowser/config/configinit.py
+++ b/qutebrowser/config/configinit.py
@@ -199,6 +199,90 @@ def qt_args(namespace: argparse.Namespace) -> typing.List[str]:
     return argv
 
 
+def _darkmode_settings() -> typing.Iterator[typing.Tuple[str, str]]:
+    """Get necessary blink settings to configure dark mode for QtWebEngine."""
+    if not config.val.colors.webpage.darkmode.enabled:
+        return
+
+    # Mapping from a colors.webpage.darkmode.algorithm setting value to
+    # Chromium's DarkModeInversionAlgorithm enum values.
+    algorithms = {
+        # 0: kOff (not exposed)
+        # 1: kSimpleInvertForTesting (not exposed)
+        'brightness-rgb': 2,  # kInvertBrightness
+        'lightness-hsl': 3,  # kInvertLightness
+        'lightness-cielab': 4,  # kInvertLightnessLAB
+    }
+
+    # Mapping from a colors.webpage.darkmode.policy.images setting value to
+    # Chromium's DarkModeImagePolicy enum values.
+    image_policies = {
+        'always': 0,  # kFilterAll
+        'never': 1,  # kFilterNone
+        'smart': 2,  # kFilterSmart
+    }
+
+    # Mapping from a colors.webpage.darkmode.policy.page setting value to
+    # Chromium's DarkModePagePolicy enum values.
+    page_policies = {
+        'always': 0,  # kFilterAll
+        'smart': 1,  # kFilterByBackground
+    }
+
+    bools = {
+        True: 'true',
+        False: 'false',
+    }
+
+    _setting_description_type = typing.Tuple[
+        str,  # qutebrowser option name
+        str,  # darkmode setting name
+        # Mapping from the config value to a string (or something convertable
+        # to a string) which gets passed to Chromium.
+        typing.Optional[typing.Mapping[typing.Any, typing.Union[str, int]]],
+    ]
+    if qtutils.version_check('5.15', compiled=False):
+        settings = [
+            ('enabled', 'Enabled', bools),
+            ('algorithm', 'InversionAlgorithm', algorithms),
+        ]  # type: typing.List[_setting_description_type]
+        mandatory_setting = 'enabled'
+    else:
+        settings = [
+            ('algorithm', '', algorithms),
+        ]
+        mandatory_setting = 'algorithm'
+
+    settings += [
+        ('contrast', 'Contrast', None),
+        ('policy.images', 'ImagePolicy', image_policies),
+        ('policy.page', 'PagePolicy', page_policies),
+        ('threshold.text', 'TextBrightnessThreshold', None),
+        ('threshold.background', 'BackgroundBrightnessThreshold', None),
+        ('grayscale.all', 'Grayscale', bools),
+        ('grayscale.images', 'ImageGrayscale', None),
+    ]
+
+    for setting, key, mapping in settings:
+        # To avoid blowing up the commandline length, we only pass modified
+        # settings to Chromium, as our defaults line up with Chromium's.
+        # However, we always pass enabled/algorithm to make sure dark mode gets
+        # actually turned on.
+        value = config.instance.get(
+            'colors.webpage.darkmode.' + setting,
+            fallback=setting == mandatory_setting)
+        if isinstance(value, usertypes.Unset):
+            continue
+
+        if mapping is not None:
+            value = mapping[value]
+
+        # FIXME: This is "forceDarkMode" starting with Chromium 83
+        prefix = 'darkMode'
+
+        yield prefix + key, str(value)
+
+
 def _qtwebengine_args(namespace: argparse.Namespace) -> typing.Iterator[str]:
     """Get the QtWebEngine arguments to use based on the config."""
     if not qtutils.version_check('5.11', compiled=False):
@@ -224,6 +308,11 @@ def _qtwebengine_args(namespace: argparse.Namespace) -> typing.Iterator[str]:
         yield '--enable-logging'
         yield '--v=1'
 
+    blink_settings = list(_darkmode_settings())
+    if blink_settings:
+        yield '--blink-settings=' + ','.join('{}={}'.format(k, v)
+                                             for k, v in blink_settings)
+
     settings = {
         'qt.force_software_rendering': {
             'software-opengl': None,
diff --git a/scripts/dev/misc_checks.py b/scripts/dev/misc_checks.py
index 24c3a1ddc..6bf411bba 100644
--- a/scripts/dev/misc_checks.py
+++ b/scripts/dev/misc_checks.py
@@ -91,7 +91,7 @@ def check_spelling():
              '[Mm]ininum', '[Rr]esett?ed', '[Rr]ecieved', '[Rr]egularily',
              '[Uu]nderlaying', '[Ii]nexistant', '[Ee]lipsis', 'commiting',
              'existant', '[Rr]esetted', '[Ss]imilarily', '[Ii]nformations',
-             '[Aa]n [Uu][Rr][Ll]'}
+             '[Aa]n [Uu][Rr][Ll]', '[Tt]reshold'}
 
     # Words which look better when splitted, but might need some fine tuning.
     words |= {'[Ww]ebelements', '[Mm]ouseevent', '[Kk]eysequence',
diff --git a/tests/unit/config/test_configinit.py b/tests/unit/config/test_configinit.py
index 731c62a66..25c6ce85e 100644
--- a/tests/unit/config/test_configinit.py
+++ b/tests/unit/config/test_configinit.py
@@ -28,7 +28,7 @@ import pytest
 from qutebrowser import qutebrowser
 from qutebrowser.config import (config, configexc, configfiles, configinit,
                                 configdata, configtypes)
-from qutebrowser.utils import objreg, usertypes
+from qutebrowser.utils import objreg, usertypes, version
 from helpers import utils
 
 
@@ -705,6 +705,120 @@ class TestQtArgs:
 
         assert ('--force-dark-mode' in args) == added
 
+    def test_blink_settings(self, config_stub, monkeypatch, parser):
+        monkeypatch.setattr(configinit.objects, 'backend',
+                            usertypes.Backend.QtWebEngine)
+        monkeypatch.setattr(configinit.qtutils, 'version_check',
+                            lambda version, exact=False, compiled=True:
+                            True)
+
+        config_stub.val.colors.webpage.darkmode.enabled = True
+
+        parsed = parser.parse_args([])
+        args = configinit.qt_args(parsed)
+
+        assert '--blink-settings=darkModeEnabled=true' in args
+
+
+class TestDarkMode:
+
+    @pytest.mark.parametrize('settings, new_qt, expected', [
+        # Disabled
+        ({}, True, []),
+        ({}, False, []),
+
+        # Enabled without customization
+        (
+            {'enabled': True},
+            True,
+            [('darkModeEnabled', 'true')]
+        ),
+        (
+            {'enabled': True},
+            False,
+            [('darkMode', '4')]
+        ),
+
+        # Algorithm
+        (
+            {'enabled': True, 'algorithm': 'brightness-rgb'},
+            True,
+            [('darkModeEnabled', 'true'),
+             ('darkModeInversionAlgorithm', '2')],
+        ),
+        (
+            {'enabled': True, 'algorithm': 'brightness-rgb'},
+            False,
+            [('darkMode', '2')],
+        ),
+
+    ])
+    @utils.qt514
+    def test_basics(self, config_stub, monkeypatch,
+                    settings, new_qt, expected):
+        for k, v in settings.items():
+            config_stub.set_obj('colors.webpage.darkmode.' + k, v)
+        monkeypatch.setattr(configinit.qtutils, 'version_check',
+                            lambda version, exact=False, compiled=True:
+                            new_qt)
+
+        assert list(configinit._darkmode_settings()) == expected
+
+    @pytest.mark.parametrize('setting, value, exp_key, exp_val', [
+        ('contrast', -0.5,
+         'darkModeContrast', '-0.5'),
+        ('policy.page', 'smart',
+         'darkModePagePolicy', '1'),
+        ('policy.images', 'smart',
+         'darkModeImagePolicy', '2'),
+        ('threshold.text', 100,
+         'darkModeTextBrightnessThreshold', '100'),
+        ('threshold.background', 100,
+         'darkModeBackgroundBrightnessThreshold', '100'),
+        ('grayscale.all', True,
+         'darkModeGrayscale', 'true'),
+        ('grayscale.images', 0.5,
+         'darkModeImageGrayscale', '0.5'),
+    ])
+    def test_customization(self, config_stub, monkeypatch,
+                           setting, value, exp_key, exp_val):
+        config_stub.val.colors.webpage.darkmode.enabled = True
+        config_stub.set_obj('colors.webpage.darkmode.' + setting, value)
+        monkeypatch.setattr(configinit.qtutils, 'version_check',
+                            lambda version, exact=False, compiled=True:
+                            True)
+
+        expected = [('darkModeEnabled', 'true'), (exp_key, exp_val)]
+        assert list(configinit._darkmode_settings()) == expected
+
+    @utils.qt514
+    def test_new_chromium(self):
+        """Fail if we encounter an unknown Chromium version.
+
+        Dark mode in Chromium currently is undergoing various changes (as it's
+        relatively recent), and Qt 5.15 is supposed to update the underlying
+        Chromium at some point.
+
+        Make this test fail deliberately with newer Chromium versions, so that
+        we can test whether dark mode still works manually, and adjust if not.
+        """
+        assert version._chromium_version() in [
+            'unavailable',  # QtWebKit
+            '77.0.3865.129',  # Qt 5.14
+            '80.0.3987.163',  # Qt 5.15
+        ]
+
+    def test_options(self, configdata_init):
+        """Make sure all darkmode options have the right attributes set."""
+        for name, opt in configdata.DATA.items():
+            if not name.startswith('colors.webpage.darkmode.'):
+                continue
+
+            backends = {'QtWebEngine': 'Qt 5.14', 'QtWebKit': False}
+            assert not opt.supports_pattern, name
+            assert opt.restart, name
+            assert opt.raw_backends == backends, name
+
 
 @pytest.mark.parametrize('arg, confval, used', [
     # overridden by commandline arg