Have a look at this option from the Mendix appstore: https://appstore.home.mendix.com/link/app/51520/Bizzomate/CSS-Loader
From your question I gather that this could fit your needs.
You can also check out the style sheet switcher widget. You can change the style sheet based on a microflow.
https://appstore.home.mendix.com/link/app/106033/TimeSeries/StyleSheetSwitcher