WindowServer: Reuse config variable from the class on theme change

When changing the theme, there were two Core::ConfigFile instances
(one class scoped -- m_config and one function scoped -- wm_config)
fighting over the file, resulting in not saving the new theme name
to the config. :^(

This makes WindowServer remember selected theme from the menu
after reboot!

Userland/Services/WindowServer/WindowManager.cpp

@@ -1955,10 +1955,9 @@ bool WindowManager::update_theme(String theme_path, String theme_name)
         return false;
     Gfx::set_system_theme(new_theme);
     m_palette = Gfx::PaletteImpl::create_with_anonymous_buffer(new_theme);
-    auto wm_config = Core::ConfigFile::open("/etc/WindowServer.ini");
-    wm_config->write_entry("Theme", "Name", theme_name);
-    wm_config->remove_entry("Background", "Color");
-    wm_config->sync();
+    m_config->write_entry("Theme", "Name", theme_name);
+    m_config->remove_entry("Background", "Color");
+    m_config->sync();
     invalidate_after_theme_or_font_change();
     return true;
 }