Make CSS editor "inactive" message more explicite

When a branding is not marked as the default one, a message is displayed on the CSS editor tab that says : "The CSS included by the default theme can only be customized on the branding marked as default." Since user can still type content in the editor, maybe the message could more explicit and say : "Only the default branding's CSS can be customized. Set this branding as default in the Common tab to customize the CSS."