Create a theme handling system for UI
Before developpings IG windows systems, we should have the possibility to switch UI themes.
The documentation is here : https://docs.godotengine.org/en/latest/tutorials/ui/gui_skinning.html and https://docs.godotengine.org/en/stable/classes/class_theme.html
I (YannK) could work on the themes themselves (studying the way to have a TODO list to create new ones), but I would need the selector in place beforehand.
There should be the choice between two themes : lakne & zbasu (which are the 2 energies of the Khanat)