Where can I find all the themes in \usefonttheme?

If you're looking for all possible font themes you can use, or looking for what's in those themes becaue you want to roll your own, you won't find this in the definition of \userfonttheme.

The themes are defined in their own files named beamerfonttheme‹themename›.sty; beamer's own come in a subdirectory themes/font, but that's not neccessary: \usefonttheme is a very thin wrapper around \usepackage.

FWIW, the font themes that currently come with beamer (i.e. files in $TEXMF/latex/beamer/themes/font) are:

• default
• serif
• professionalfonts
• structurebold
• structureitalicserif
• structuresmallcapsserif

and they are documented in section 18.1 of the userguide.

If you want to see different combinations of Beamer in action, here you can switch and see how different combinations of style, color and fonts in LaTeX looks like: http://deic.uab.es/~iblanes/beamer_gallery/individual/default-default-default.html

This beamer theme matrix allows you to figure out different combo of theme and font

https://hartwork.org/beamer-theme-matrix/