feat: Add getAccentColor on Linux (#48628)

* feat: Implement `getAccentColor` on Linux

Co-authored-by: Tau Gärtli <git@tau.garden>

* doc: Update OS support for accent color APIs

Co-authored-by: Tau Gärtli <git@tau.garden>

---------

Co-authored-by: trop[bot] <37223003+trop[bot]@users.noreply.github.com>
Co-authored-by: Tau Gärtli <git@tau.garden>

shell/browser/api/electron_api_system_preferences.h

@@ -18,6 +18,11 @@
 #include "shell/browser/browser.h"
 #include "shell/browser/browser_observer.h"
 #endif
+#if BUILDFLAG(IS_LINUX)
+#include "base/memory/raw_ptr.h"
+#include "ui/native_theme/native_theme.h"
+#include "ui/native_theme/native_theme_observer.h"
+#endif
 
 namespace gin_helper {
 template <typename T>
@@ -44,6 +49,9 @@ class SystemPreferences final
 #if BUILDFLAG(IS_WIN)
     ,
       public BrowserObserver
+#elif BUILDFLAG(IS_LINUX)
+    ,
+      public ui::NativeThemeObserver
 #endif
 {
  public:
@@ -55,8 +63,8 @@ class SystemPreferences final
       v8::Isolate* isolate) override;
   const char* GetTypeName() override;
 
+  std::string GetAccentColor();
 #if BUILDFLAG(IS_WIN) || BUILDFLAG(IS_MAC)
-  static std::string GetAccentColor();
   std::string GetColor(gin_helper::ErrorThrower thrower,
                        const std::string& color);
   std::string GetMediaAccessStatus(gin_helper::ErrorThrower thrower,
@@ -117,6 +125,10 @@ class SystemPreferences final
   // TODO(MarshallOfSound): Write tests for these methods once we
   // are running tests on a Mojave machine
   v8::Local<v8::Value> GetEffectiveAppearance(v8::Isolate* isolate);
+
+#elif BUILDFLAG(IS_LINUX)
+  // ui::NativeThemeObserver:
+  void OnNativeThemeUpdated(ui::NativeTheme* theme) override;
 #endif
   v8::Local<v8::Value> GetAnimationSettings(v8::Isolate* isolate);
 
@@ -162,6 +174,12 @@ class SystemPreferences final
   // Color/high contrast mode change observer.
   base::CallbackListSubscription hwnd_subscription_;
 #endif
+#if BUILDFLAG(IS_LINUX)
+  void OnNativeThemeUpdatedOnUI();
+
+  raw_ptr<ui::NativeTheme> ui_theme_;
+  std::string current_accent_color_;
+#endif
 };
 
 }  // namespace electron::api