Update web_instance on GTK when checking for dark mode

Adds some extra code to change the theme in the web instance when on GTK,
to fix prefers-color-scheme.

Also makes SystemDarkModeSupported return true on Linux,
since the most modern Linux distributions support it,
and Linux dark mode code doesn’t detect whether or not it is supported.
This function seems to only be used in telemetry, so maybe it doesn’t
need to be changed.

Bug: 998903
Change-Id: I02a084d9f733b01c8e6ad0f5ca5f1ebcfb16d475
diff --git a/AUTHORS b/AUTHORS
index 758d0bf..8c91146 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -735,6 +735,7 @@
 Leith Bade <leith@leithalweapon.geek.nz>
 Lei Gao <leigao@huawei.com>
 Lei Li <lli.kernel.kvm@gmail.com>
+Lena Wildervanck <lena.wildervanck@gmail.com>
 Lenny Khazan <lenny.khazan@gmail.com>
 Leo Wolf <jclw@ymail.com>
 Leon Han <leon.han@intel.com>
diff --git a/chrome/common/chrome_features.cc b/chrome/common/chrome_features.cc
index 85c3931..2b0c2a84 100644
--- a/chrome/common/chrome_features.cc
+++ b/chrome/common/chrome_features.cc
@@ -1430,12 +1430,12 @@
 BASE_FEATURE(kWebUIDarkMode,
              "WebUIDarkMode",
 #if BUILDFLAG(IS_MAC) || BUILDFLAG(IS_WIN) || BUILDFLAG(IS_ANDROID) || \
-    BUILDFLAG(IS_CHROMEOS)
+    BUILDFLAG(IS_CHROMEOS) || BUILDFLAG(IS_LINUX)
              base::FEATURE_ENABLED_BY_DEFAULT
 #else
              base::FEATURE_DISABLED_BY_DEFAULT
 #endif  // BUILDFLAG(IS_MAC) || BUILDFLAG(IS_WIN) || BUILDFLAG(IS_ANDROID) ||
-        // BUILDFLAG(IS_CHROMEOS)
+        // BUILDFLAG(IS_CHROMEOS) || BUILDFLAG(IS_LINUX)
 );
 
 #if BUILDFLAG(IS_CHROMEOS_ASH)
diff --git a/ui/gtk/native_theme_gtk.cc b/ui/gtk/native_theme_gtk.cc
index a60d6e3..1200a80 100644
--- a/ui/gtk/native_theme_gtk.cc
+++ b/ui/gtk/native_theme_gtk.cc
@@ -165,6 +165,14 @@
       UserHasContrastPreference()
           ? ui::NativeThemeBase::PreferredContrast::kMore
           : ui::NativeThemeBase::PreferredContrast::kNoPreference);
+
+  // Update the web instance too to fix prefers-color-theme
+  NativeTheme* web_instance = NativeTheme::GetInstanceForWeb();
+  web_instance->set_use_dark_colors(ShouldUseDarkColors());
+  web_instance->set_preferred_color_scheme(CalculatePreferredColorScheme());
+  web_instance->SetPreferredContrast(CalculatePreferredContrast());
+  web_instance->NotifyOnNativeThemeUpdated();
+
   native_theme->NotifyOnNativeThemeUpdated();
 }
 
diff --git a/ui/native_theme/native_theme.cc b/ui/native_theme/native_theme.cc
index 835fa36..feec109 100644
--- a/ui/native_theme/native_theme.cc
+++ b/ui/native_theme/native_theme.cc
@@ -36,7 +36,11 @@
 #if !BUILDFLAG(IS_WIN) && !BUILDFLAG(IS_APPLE)
 // static
 bool NativeTheme::SystemDarkModeSupported() {
+#if BUILDFLAG(IS_LINUX)
+  return true;
+#else
   return false;
+#endif
 }
 #endif