Please note: the DHTMLx tree you show in your screenshot will be replaced with a Shoelace web-component from the next 23.1 maintenance release on. It’s already in the 23.1 branch on Github. So don’t waste your time changing what’s already gone.
Generally speaking: EGroupware has themes created by .css files generated .less files in the following directory:
It makes sense to create your own .less file in that directory including the pixelegg.less file, as does the mobile.less file for example, and then overwrite what you want to change.
The next maintenance release (current 23.1 branch on Github) already includes the ability to specify a custom (web-)font to be used EGroupware wide.
Ralf