Yes, you can change color, component size using css.
in your project Directory-> theme\styles\web\sass\app you can find custome.scss file
in that you change all the things.
in the Mendix UI theme you only can change colors and logos. However you can access to the porject directory to edit front-end parts more indeep.
In this learning path: https://gettingstarted.mendixcloud.com/link/module/212/lecture/1646/3.1-Introduction you can find more info that can help you.
Hi Nada ,
have look on this doc below
and u can get idea about this https://gettingstarted.mendixcloud.com/link/path/61/Unlocking-True-Collaboration