CSS for anything?

This would actually depend on the theme you are using. And usually, the fonts are changed via the theme. Also we have a guide on our site on how to customize a knowledge base theme

Via theme:
Modern Theme (ex.) > Google Font @import > @import url(‘https://fonts.googleapis.com/css2?family=Overlock&display=swap’); [or other font – go Google Fonts, choose and copy “@import url…”)