Custom cpanel / roundcube theme install

ok i have already created a custom theme for roundcube it works. BUT when cpanel updates it updates roundcube also. when it updates roundcube any custom themes get deleted! making files immutable does not work! I need someone who knows how to use roundcube overlay files. I tried but i cannot get it to work! the theme is complete - i just need to set up the overlay file so every time cpanel updates it does not remove the custom theme in roundcube!