Issue #1752 - Implement "prefers-color-scheme" as a user preference.

This PR passes all current tests for this feature, and implements the "prefers-color-scheme" media query as an enumerated keyword that is controlled by an integer preference.

Currently, the possible options are 0 to see a website's fallback code and essentially behave like this isn't implemented (our current behavior), 1 to express a preference for a light theme (the default for spec reasons), and 2 to express a preference for a dark theme. Over time, this list may expand to include other preferences like a preference for a sepia theme or something, and this leaves us prepared for that future.

dom/base/nsGkAtomList.h

@@ -2240,6 +2240,7 @@ GK_ATOM(scrollbar_end_backward, "scrollbar-end-backward")
 GK_ATOM(scrollbar_end_forward, "scrollbar-end-forward")
 GK_ATOM(scrollbar_thumb_proportional, "scrollbar-thumb-proportional")
 GK_ATOM(overlay_scrollbars, "overlay-scrollbars")
+GK_ATOM(prefers_color_scheme, "prefers-color-scheme")
 GK_ATOM(windows_accent_color_applies, "windows-accent-color-applies")
 GK_ATOM(windows_accent_color_is_dark, "windows-accent-color-is-dark")
 GK_ATOM(windows_default_theme, "windows-default-theme")

layout/style/nsCSSKeywordList.h

@@ -224,6 +224,7 @@ CSS_KEY(crosshair, crosshair)
 CSS_KEY(currentcolor, currentcolor)
 CSS_KEY(cursive, cursive)
 CSS_KEY(cyclic, cyclic)
+CSS_KEY(dark, dark)
 CSS_KEY(darken, darken)
 CSS_KEY(dashed, dashed)
 CSS_KEY(dense, dense)
@@ -364,6 +365,7 @@ CSS_KEY(last baseline, last_baseline) // only used for DevTools auto-completion
 CSS_KEY(layout, layout)
 CSS_KEY(left, left)
 CSS_KEY(legacy, legacy)
+CSS_KEY(light, light)
 CSS_KEY(lighten, lighten)
 CSS_KEY(lighter, lighter)
 CSS_KEY(line-through, line_through)

layout/style/nsMediaFeatures.cpp

@@ -20,6 +20,7 @@
 #include "nsIDocument.h"
 #include "nsIWidget.h"
 #include "nsContentUtils.h"
+#include "mozilla/Preferences.h"
 #include "mozilla/StyleSheet.h"
 #include "mozilla/StyleSheetInlines.h"
 
@@ -45,6 +46,12 @@ static const nsCSSProps::KTableEntry kDisplayModeKeywords[] = {
   { eCSSKeyword_UNKNOWN,                 -1 }
 };
 
+static const nsCSSProps::KTableEntry kPrefersColorSchemeKeywords[] = {
+  { eCSSKeyword_light,			 NS_STYLE_PREFERS_COLOR_SCHEME_LIGHT },
+  { eCSSKeyword_dark,			 NS_STYLE_PREFERS_COLOR_SCHEME_DARK },
+  { eCSSKeyword_UNKNOWN,		 -1 },
+};
+
 #ifdef XP_WIN
 struct WindowsThemeName {
   LookAndFeel::WindowsTheme id;
@@ -457,6 +464,25 @@ GetOperatingSystemVersion(nsPresContext* aPresContext, const nsMediaFeature* aFe
   return NS_OK;
 }
 
+static nsresult
+GetPrefersColorScheme(nsPresContext* aPresContext, const nsMediaFeature* aFeature,
+		      nsCSSValue& aResult)
+{
+  switch(Preferences::GetInt("browser.display.prefers_color_scheme", 1)) {
+    case 1:
+    aResult.SetIntValue(NS_STYLE_PREFERS_COLOR_SCHEME_LIGHT, 
+		        eCSSUnit_Enumerated);
+    break;
+    case 2:
+    aResult.SetIntValue(NS_STYLE_PREFERS_COLOR_SCHEME_DARK, 
+		        eCSSUnit_Enumerated);
+    break;
+    default:
+    aResult.Reset();
+  }
+  return NS_OK;
+}
+
 static nsresult
 GetIsGlyph(nsPresContext* aPresContext, const nsMediaFeature* aFeature,
           nsCSSValue& aResult)
@@ -556,6 +582,14 @@ nsMediaFeatures::features[] = {
     { nullptr },
     GetMonochrome
   },
+  {
+    &nsGkAtoms::prefers_color_scheme,
+    nsMediaFeature::eMinMaxNotAllowed,
+    nsMediaFeature::eEnumerated,
+    nsMediaFeature::eNoRequirements,
+    { kPrefersColorSchemeKeywords },
+    GetPrefersColorScheme
+  },
   {
     &nsGkAtoms::resolution,
     nsMediaFeature::eMinMaxAllowed,

layout/style/nsStyleConsts.h

@@ -1306,6 +1306,10 @@ enum class StyleDisplay : uint8_t {
 #define NS_STYLE_DISPLAY_MODE_STANDALONE        2
 #define NS_STYLE_DISPLAY_MODE_FULLSCREEN        3
 
+// prefers-color-scheme
+#define NS_STYLE_PREFERS_COLOR_SCHEME_LIGHT	0
+#define NS_STYLE_PREFERS_COLOR_SCHEME_DARK	1
+
 } // namespace mozilla
 
 #endif /* nsStyleConsts_h___ */

modules/libpref/init/all.js

@@ -266,6 +266,10 @@ pref("browser.display.use_document_fonts",  1);  // 0 = never, 1 = quick, 2 = al
 // 1 = always
 // 2 = never
 pref("browser.display.document_color_use", 0);
+// 0 = feature disabled
+// 1 = default: light theme preferred
+// 2 = dark theme preferred
+pref("browser.display.prefers_color_scheme", 1);
 pref("browser.display.use_system_colors",   false);
 pref("browser.display.foreground_color",    "#000000");
 pref("browser.display.background_color",    "#FFFFFF");