Oh that… well making a theme isn’t that hard to do. However removing functions that are in the current one isn’t a good idea as most tie into other parts of virtualmin and could possibly break something.
It is also good for me if the most important functions are highlighted in my language. The default themes are really complicated to an everage user. There are too many functions the users don’t know how/what to use for. That is why I had the idea to make my own theme where only the frequently used functions are with explanations in my language. I thought if I write my explanations into the theme it won’t be lost with the updates.
I’m not sure what features/links in the Virtualmin theme are confusing. The only difference between VM Pro theme and say cpanel is the lack of a cluttered main screen with icons.
There are the docs for VM Pro/GPL as well. On the main screen top right there is a link to the docs, you could just translate those into your own language and past them to your clients as needed.