On cPanel Interface, under 'Change Look and Feel' feature, you may find the X3 theme on it. If you can't find it, you need to contact your provider to switch it back.

If you are a provider, you can configure it to display X3 theme on the theme list by going to WHM (root) -> RVGlobalSoft Manager -> RVSkin Manager -> Themes -> Selectable Themes, and make sure the X3 is checked on the other theme section.