if I add new fonts in the theme it would be visible in a Label node's Theme Overrides
It wouldn't. Theme (which is currently in effect for a particular
Control) is used for default appearance for all items of given class (e.g. label). Control's Theme Overrides is used when you want to override some properties for a particular instance. Final appearance id defined by a cascaded hierarchy, that looks complicated, but actually is very flexible and resembles CSS principles of cascading:
Built-in Godot Theme
Project's default Theme
Parent control's Theme
Control's Theme Overrides
Personally, I'm mostly using some small themes like "Red label" with just some properties (like font, color) redefined for Label class, which is later assigned (as Theme) for a given Control instance.
p.s. Revamped editor is just it: editor, not theme engine.