@mitu Forgot about this! Thanks for the reply! You're right the theme works fine with RetroPie 4.8 on the download page. I read that the theme would break....I don't dare update 4.8 though! (I may DD it beforehand and then try!).
Since you asked which theme it is (but it might be a little sketchy):
I certainly don't expect any support here for it.
Also when I installed 4.8 the slowdown when using shaders is much reduced and actually tolerable now. Happy days!
Next item on the agenda is to read up on overclocking the Pi3B+ because I wouldn't mind having run-ahead enabled.
Cheers mate!