Check out the documentation:
One method is to create an additional css file (custom.css for example) and import that in your app's index.html after the other style sheets. You can then add further styling rules in that file and even overload some of the definitions in the theme.
Another method is to modify files in the theme zip directly, but this will make it harder to track which parts are your original theme and which files you have customized.
Perhaps you can find your answers on: https://ux.mendix.com/theme-viewer.html?profile=dekstop#!/MyFirstModule/How_to_customize.