DisplaySettings: Reapply old fix to avoid resetting background color

A reapplication of fix from bf9c08c,
removed in e9e4bae. This meant
changing an unrelated setting could change the background color to the
current theme's default.

Author: MacDue

Userland/Applications/DisplaySettings/ThemesSettingsWidget.cpp

@@ -163,6 +163,11 @@ ThemesSettingsWidget::ThemesSettingsWidget(bool& background_settings_changed)
 {
 }
 
+static inline ByteString current_system_theme()
+{
+    return GUI::ConnectionToWindowServer::the().get_system_theme();
+}
+
 void ThemesSettingsWidget::apply_settings()
 {
     auto color_scheme_path_or_error = String::formatted("/res/color-schemes/{}.ini", m_selected_color_scheme_name);
@@ -172,12 +177,13 @@ void ThemesSettingsWidget::apply_settings()
     }
     auto color_scheme_path = color_scheme_path_or_error.release_value();
 
+    bool keep_background_color = m_background_settings_changed || (m_selected_theme && m_selected_theme->name == current_system_theme());
     if (!m_color_scheme_is_file_based && find_descendant_of_type_named<GUI::CheckBox>("custom_color_scheme_checkbox")->is_checked()) {
-        auto set_theme_result = GUI::ConnectionToWindowServer::the().set_system_theme(m_selected_theme->path, m_selected_theme->name, m_background_settings_changed, "Custom"sv);
+        auto set_theme_result = GUI::ConnectionToWindowServer::the().set_system_theme(m_selected_theme->path, m_selected_theme->name, keep_background_color, "Custom"sv);
         if (!set_theme_result)
             GUI::MessageBox::show_error(window(), "Failed to apply theme settings"sv);
     } else if (find_descendant_of_type_named<GUI::CheckBox>("custom_color_scheme_checkbox")->is_checked()) {
-        auto set_theme_result = GUI::ConnectionToWindowServer::the().set_system_theme(m_selected_theme->path, m_selected_theme->name, m_background_settings_changed, color_scheme_path.to_byte_string());
+        auto set_theme_result = GUI::ConnectionToWindowServer::the().set_system_theme(m_selected_theme->path, m_selected_theme->name, keep_background_color, color_scheme_path.to_byte_string());
         if (!set_theme_result)
             GUI::MessageBox::show_error(window(), "Failed to apply theme settings"sv);
     } else {
@@ -187,7 +193,7 @@ void ThemesSettingsWidget::apply_settings()
             return;
         }
         auto preferred_color_scheme_path = get_color_scheme_name_from_pathname(theme_config.release_value()->read_entry("Paths", "ColorScheme"));
-        auto set_theme_result = GUI::ConnectionToWindowServer::the().set_system_theme(m_selected_theme->path, m_selected_theme->name, m_background_settings_changed, OptionalNone());
+        auto set_theme_result = GUI::ConnectionToWindowServer::the().set_system_theme(m_selected_theme->path, m_selected_theme->name, keep_background_color, OptionalNone());
         if (!set_theme_result || preferred_color_scheme_path.is_error()) {
             GUI::MessageBox::show_error(window(), "Failed to apply theme settings"sv);
             return;