How do I change the entire theme of an Ionic app, without SASS?

I’m making an app that uses a white text on coloured header, background, colour themed tabs and list items that are the menu, just like in the default Ionic starter.

I can change the colour things by hacking in custom css and lots of !important but this can’t really be the right way to do things. I don’t want to learn or install SASS just for what should be something really simple.

Is there any way to do this?