Theme not showing up in-game

Hey everyone!

I've recently started a game in Godot and now wanted to create a bare-bones main menu for it.

I made a main_menu.tres theme so that I can centralize the controls for the theme.

I applied the theme to the MainMenu UI Node all the way at the top of the scene.

However, when I run the game, the theme is not being applied:

My buttons are themed using these FlatBoxThemes:

Running Godot 4.2.2

Thanks in advance!