Issue #1784 - Add -moz-dark-theme media query and allow prefers-color-scheme to follow it.

dom/base/nsGkAtomList.h

@@ -2267,6 +2267,7 @@ GK_ATOM(windows_theme_generic, "windows-theme-generic")
 
 // And the same again, as media query keywords.
 GK_ATOM(_moz_color_picker_available, "-moz-color-picker-available")
+GK_ATOM(_moz_dark_theme, "-moz-dark-theme")
 GK_ATOM(_moz_scrollbar_start_backward, "-moz-scrollbar-start-backward")
 GK_ATOM(_moz_scrollbar_start_forward, "-moz-scrollbar-start-forward")
 GK_ATOM(_moz_scrollbar_end_backward, "-moz-scrollbar-end-backward")

layout/style/nsMediaFeatures.cpp

@@ -466,20 +466,60 @@ GetOperatingSystemVersion(nsPresContext* aPresContext, const nsMediaFeature* aFe
 
 static nsresult
 GetPrefersColorScheme(nsPresContext* aPresContext, const nsMediaFeature* aFeature,
-		      nsCSSValue& aResult)
+          nsCSSValue& aResult)
 {
   switch(Preferences::GetInt("browser.display.prefers_color_scheme", 1)) {
     case 1:
-    aResult.SetIntValue(NS_STYLE_PREFERS_COLOR_SCHEME_LIGHT, 
+      aResult.SetIntValue(NS_STYLE_PREFERS_COLOR_SCHEME_LIGHT, 
-		        eCSSUnit_Enumerated);
+                          eCSSUnit_Enumerated);
-    break;
+      break;
     case 2:
-    aResult.SetIntValue(NS_STYLE_PREFERS_COLOR_SCHEME_DARK, 
+      aResult.SetIntValue(NS_STYLE_PREFERS_COLOR_SCHEME_DARK, 
-		        eCSSUnit_Enumerated);
+                          eCSSUnit_Enumerated);
-    break;
+      break;
+    case 3:
+      // If the pref is 3, we follow ui.color_scheme instead. When following 
+      // ui.color_scheme, light theme is the fallback behavior.
+      switch(Preferences::GetInt("ui.color_scheme", 1)) {
+        case 2:
+          aResult.SetIntValue(NS_STYLE_PREFERS_COLOR_SCHEME_DARK,
+                              eCSSUnit_Enumerated);
+          break;
+        default:
+          aResult.SetIntValue(NS_STYLE_PREFERS_COLOR_SCHEME_LIGHT,
+                              eCSSUnit_Enumerated);
+      }
+      break;
     default:
-    aResult.Reset();
+      aResult.Reset();
   }
+
+  return NS_OK;
+}
+
+static nsresult
+GetDarkTheme(nsPresContext* aPresContext, const nsMediaFeature* aFeature,
+             nsCSSValue& aResult)
+{
+#ifdef XP_WIN
+  // Under Windows, do nothing if High Contrast Theme is on.
+  if (LookAndFeel::GetInt(LookAndFeel::eIntID_UseAccessibilityTheme, 0)) {
+    aResult.Reset();
+    return NS_OK;
+  }
+#endif
+
+  switch(Preferences::GetInt("ui.color_scheme", 1)) {
+    case 1:
+      aResult.SetIntValue(0, eCSSUnit_Integer);
+      break;
+    case 2:
+      aResult.SetIntValue(1, eCSSUnit_Integer);
+      break;
+    default:
+      aResult.Reset();
+  }
+
   return NS_OK;
 }
 
@@ -645,6 +685,14 @@ nsMediaFeatures::features[] = {
   },
 
   // Mozilla extensions
+  {
+    &nsGkAtoms::_moz_dark_theme,
+    nsMediaFeature::eMinMaxNotAllowed,
+    nsMediaFeature::eBoolInteger,
+    nsMediaFeature::eNoRequirements,
+    { nullptr },
+    GetDarkTheme
+  },
   {
     &nsGkAtoms::_moz_device_pixel_ratio,
     nsMediaFeature::eMinMaxAllowed,

modules/libpref/init/all.js

@@ -250,6 +250,13 @@ pref("browser.sessionhistory.max_total_viewers", -1);
 // See https://github.com/MoonchildProductions/UXP/issues/719
 pref("browser.newtabpage.add_to_session_history", false);
 
+
+// Determines whether the browser's current theme should be light or dark.
+// 0 = feature disabled
+// 1 = default: light theme
+// 2 = dark theme
+pref("ui.color_scheme", 1);
+
 pref("ui.use_native_colors", true);
 #ifdef MOZ_WIDGET_GTK
 // Determines whether the menubar is shown in the global menubar or not.
@@ -269,6 +276,7 @@ pref("browser.display.document_color_use", 0);
 // 0 = feature disabled
 // 1 = default: light theme preferred
 // 2 = dark theme preferred
+// 3 = match ui.color_scheme
 pref("browser.display.prefers_color_scheme", 1);
 pref("browser.display.use_system_colors",   false);
 pref("browser.display.foreground_color",    "#000000");