gui, widgets: Use system colors on Windows title bar...

...when System colors theme is set.
Previously, we just checked if the theme
was set to Light or not and used that to
decide the title bar colors. With the
System colors theme, this was no longer
valid logic.
This patch renames
themes_win32_is_darkmode_active () to
gimp_is_win32_system_theme_dark (), and
moves it to gimpwidgets-utils.c so that
it can be accessed by functions that
need to determine Windows mode
settings. We then use it to determine
dark mode if the System colors theme
is set, and use the original logic otherwise.

app/gui/themes.c

@@ -65,10 +65,6 @@ static void   themes_theme_settings_portal_changed (GDBusProxy            *proxy
                                                     Gimp                  *gimp);
 #endif
 
-#ifdef G_OS_WIN32
-static gboolean themes_win32_is_darkmode_active    (void);
-#endif
-
 
 /*  private variables  */
 
@@ -406,7 +402,7 @@ themes_apply_theme (Gimp          *gimp,
 #elif defined(G_OS_WIN32)
   if (config->theme_scheme == GIMP_THEME_SYSTEM)
     {
-      prefer_dark_theme = themes_win32_is_darkmode_active ();
+      prefer_dark_theme = gimp_is_win32_system_theme_dark ();
       color_scheme = prefer_dark_theme ? GIMP_THEME_DARK : GIMP_THEME_LIGHT;
     }
   else
@@ -874,22 +870,4 @@ themes_set_title_bar (Gimp *gimp)
   if (windows)
     g_list_free (windows);
 }
-
-static gboolean
-themes_win32_is_darkmode_active (void)
-{
-  DWORD   val      = 0;
-  DWORD   val_size = sizeof (val);
-  LSTATUS status;
-
-  status = RegGetValueA(HKEY_CURRENT_USER,
-                        "Software\\Microsoft\\Windows\\CurrentVersion\\Themes\\Personalize",
-                        "AppsUseLightTheme",
-                        RRF_RT_REG_DWORD,
-                        NULL,
-                        &val,
-                        &val_size);
-
-  return status == ERROR_SUCCESS && val == 0;
-}
 #endif

app/widgets/gimpwidgets-utils.c

@@ -2677,6 +2677,9 @@ gimp_window_set_title_bar_theme (Gimp      *gimp,
           GimpGuiConfig *config;
 
           config = GIMP_GUI_CONFIG (gimp->config);
+          if (config->theme_scheme == GIMP_THEME_SYSTEM)
+            use_dark_mode = gimp_is_win32_system_theme_dark ();
+          else
             use_dark_mode = (config->theme_scheme != GIMP_THEME_LIGHT);
         }
       else
@@ -2706,4 +2709,22 @@ gimp_window_set_title_bar_theme (Gimp      *gimp,
                                &use_dark_mode, sizeof (use_dark_mode));
     }
 }
+
+gboolean
+gimp_is_win32_system_theme_dark (void)
+{
+  DWORD   val      = 0;
+  DWORD   val_size = sizeof (val);
+  LSTATUS status;
+
+  status = RegGetValueA(HKEY_CURRENT_USER,
+                        "Software\\Microsoft\\Windows\\CurrentVersion\\Themes\\Personalize",
+                        "AppsUseLightTheme",
+                        RRF_RT_REG_DWORD,
+                        NULL,
+                        &val,
+                        &val_size);
+
+  return status == ERROR_SUCCESS && val == 0;
+}
 #endif

app/widgets/gimpwidgets-utils.h

@@ -167,6 +167,7 @@ gboolean          gimp_utils_are_menu_path_identical   (const gchar       *path1
 #ifdef G_OS_WIN32
 void              gimp_window_set_title_bar_theme      (Gimp              *gimp,
                                                         GtkWidget         *dialog);
+gboolean          gimp_is_win32_system_theme_dark      (void);
 #endif
 
 #endif /* __APP_GIMP_WIDGETS_UTILS_H__ */