Where/how to change the webmail theme/interface my endusers see

Hi, just had to move to a new cpanel server. For end users that used the webmail feature, previously I edited the old X3 mail theme so end users had minimal features to choose from (just the ones they need) and couldn't configure other ones such as forwarding to an external email address (not allowed by some of my clients). On the new server it's paper lantern by default horribly cluttered with way too much info and any useful settings hidden in a unintuitive menu top right. I am not dealing with technically competent users, this needs to be idiot proof and even then I get support queries. Can anyone point me in the right direction to clean that page up (I also want it to stick after updates). Ideally I would have the option to give my more savvy clients the full works, so does this mean a theme clone before editing so both options are available? Has someone already done this and made it available as a free theme? Thanks.