Well like stan said, we have the style.xml file concept so you can create your style file then include in your gui .xml file
https://gitea.wildfiregames.com/0ad/0ad/src/branch/main/binaries/data/mods/public/gui/common/styles.xml#L12
https://gitea.wildfiregames.com/0ad/0ad/src/branch/main/binaries/data/mods/public/gui/common/styles.xml#L66
In 0.A.D we have the page concept where you can define what styles to use (you can examine all page_*):
https://gitea.wildfiregames.com/0ad/0ad/src/branch/main/binaries/data/mods/public/gui/page_pregame.xml#L4
and after that the real GUI xml
https://gitea.wildfiregames.com/0ad/0ad/src/branch/main/binaries/data/mods/public/gui/page_pregame.xml#L15
so when we define any element we use the xml tag "Object" + attrbiute "type" to define what object... and you can apply an style with attrbiute "style"
https://gitea.wildfiregames.com/0ad/0ad/src/branch/main/binaries/data/mods/public/gui/page_pregame.xml#L15
or you can explicitly use the "font" attribute if you wan it.
Let me know if that help you :).
PD: you can test first adding the font you wanna test and replace/add the "font" tag in a particular object to see if that work, after that you can expand the idea with styles.xml and "style" attribute