If you use the mendix UI framework (or are building on a default theme) to create your theme, have a look at the demo.
Additionally, the bootstrap docs will help you.
I can really recomend using SASS. For more information I would suggest having a look at the Mendix doc
The SCSS files will show you a list of all available classes (also the nested ones, for easier implementation of your own styling).
atlas.mendix.com you can find a gallary of buildingblocks.
What class to be used to add active status on a button