Hey @Mads! At the moment the ._MZLze class should target both listboxes and select dropdowns on web. Unfortunately, custom CSS written in modules doesn't carry over to the parent app 
It is possible to add it as a CSS rule at the organization level so that it affects all of your apps. To narrow down the reach of the rule in that case it might be helpful to use a special selector, something like [id*="wrapped"], with that, the rule will only target components with wrapped in their id:
Let me know if that works!