hey thats really awesome for a new dev like myself, learning lots of tricks just by looking at the project, however i dont see how you gave the fonts to the labels when the custom font and theme are empty for them, could you explain how you did it?
oh I will fix the project in the next our :-)
But basically it should work when you click on the most parent control more inside the Game.tscn then under Theme > Custom Font > add a new Dynamic Font.
In the Dynamic Font you can select any ttf file as font file. I did use a font by accident which was free to use, but I wasn't allowed to redistribute.
Ah now I had another look at what you meant. Sorry did not read your post correctly. The font is assigned via the node UI inside the Game.tscn scene. Whenever you set a theme for a control, it will automatically be applied for all children, except when a child does have a Theme assigned by its own, then it will inherit that theme to its children aswell :-)