There are thow ways to do this you can make a new theme with difrent CSS or SASS classes but this will take a lot of time. I also had this problem recently and there is a way to dynamicly adjust CSS according to the userrole or other data see this link
Pleas contact mee if you need more info on this topic.
Thanks for your reply.
Right now i'm thinking i prefer a totaly different theme for mobile.
We have custom theme for oure desktop application and i would like to use the Mendix standard theme as base for the mobile app.
i'm going to take a look at the video link.
are there no other prefered ways of doing this?