I am following the forums to see what users request and the problems they report. I see a lot of questions that can be fixed if custom CSS and custom Javascript could be added by users. With some hacks in my browser I currently do this myself. Like moving fields around, not that difficult with custom JS. I can understand why you won’t build it, you can “break” the application with this. And of course the security risks to think about.
Custom CSS is more safe and does not have the security concerns to think about, but what you can actually can change is limited.
There are (big) websites that do allow custom CSS. Custom Javascript is quite rare.
Don’t know if you ever though about this, but I would definitely use this (even it is limited).