Create theme type for nodes created with GDExtension

[acgc99]

Describe the project you are working on

A plugin with widgets for mobile apps. I'm moving from GDScript to GDExtension.

I'm new to GDExtension so there might be a way to do so, but since there aren't online tutorials and I cannot figure out how to do that looking at Godot source code for inspiration, I'm requesting this feature.

Describe the problem or limitation you are having in your project

I would like to define theme types for the widgets for customization (it doesn't have too much sense to distribute the plugin without customization).

Describe the feature / enhancement and how it helps to overcome the problem or limitation

Adding theme types for GDExtension nodes would improve customization for plugins.

Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams

Add corresponding functions to do so.

If this enhancement will not be used often, can it be worked around with a few lines of script?

I don't think so.

Is there a reason why this should be core and not an add-on in the asset library?

This is about improving GDExtension.

[Daylily-Zeleen]

You can get the default theme by ThemeDB::get_singleton()->get_default_theme() and do some customization for your own classes.

[YuriSizov]

Hey there! This is already covered by #4486. I've recently made changes in the engine itself which formalize theme definitions for classes. We'll try to expose this to scripting and extensions by 4.3.