You might to want to do this learning path: https://academy.mendix.com/link/paths/123/Style-your-App-with-Sass It will give you insight on the workings of css and Sass. And this documentation might also help you: https://docs.mendix.com/howto/front-end/customize-styling-new/ It gives you info about the Atlas core. And this URL https://atlasdesignsystem.mendixcloud.com/p/dashboard you already play around a bit.
What Ronald says is pretty much on point. You might want to check out mFonts as well. This UI module will give you more font options right out of the gate. Also, you can inspect the stylesheets and see how it works. https://marketplace.mendix.com/link/component/206063