StackOverflow to the rescue!
This (or any for that matter) addition to the css can be easily done using sass: https://www.mxblog.nl/2016/03/using-sass-with-mendix/
This is a bug. And a big one. Modal means modal. By definition the main-window and all its elements should be inactive, untouchable, non-interfering at all for as long as the modal window is still active. So your best bet is to report a bug at Mendix and have them solve this bug. Mendix will be a platform for it.