We explored several options for manoeuvring fonts in this and other thread:
1. Color Change
The easiest and the fastest way. You change the colour code in the theme config. Example
2. Type Change
For example, for album fonts, Roon uses GrifoM-Medium natively. If we want to replace it with, for example, Arial Black, we replace the original font file with our font file (with the name of the original). So we rename the Arial Black.ttf file to GrifoM-Medium.otf. After overwriting and rebooting, Roon will display our font 
3. Size Change
Also for album fonts we can apply the operation of artificial font reduction. In our case, we follow the video and overwrite the original font file with our FontForge-generated file. What does this give us? We reduce the visible size of all sizes of a given font by 50%. Such an operation causes that when Roon displays the font 26, it actually displays 26 but with the visible size 13 because we made it smaller in the program. This is a workaround that puts an “official” font into the Roon with “official” sizes but with a visible size as we see fit.