Godot-proposals: Document how to set a Theme Property of Control node

[Error7Studios]

Describe the project you are working on:
(Applies to any project)

Describe the problem or limitation you are having in your project:
Setting a Control node's "theme property" with code is not intuitive and needs explanation in the docs.

To do this, a user has to figure out that they need to use <control>.set("custom_constants/...", <value>)
How are they supposed to figure this out?
Is the fact that you can hover over an inspector property to see its 'path' even documented anywhere?

Even if a user figures this out, when they use <control>.set(...), if they have a typo or write an incorrect string in the set(...) function, they get no warning/error to let them realize their mistake.
(See: #1494)

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

1. Document that you can hover over an inspector property to see its 'path'
2. Document how to set Control theme properties using set("custom_constants/...", <value>)

Describe how your proposal will work, with code, pseudocode, mockups, and/or diagrams:

If this enhancement will not be used often, can it be worked around with a few lines of script?:
No.

Is there a reason why this should be core and not an add-on in the asset library?:
This is a documentation-related.

P.S.
Seriously, what is the type argument in theme.set_constant(...) supposed to be?
All I could find on it was this thread from 2016.
https://godotengine.org/qa/4215/explanation-of-string-type-argument-in-theme-api

The docs page keeps using this word by never explains it.
https://docs.godotengine.org/en/stable/classes/class_theme.html

[Calinou]

We have a demo about this now :slightly_smiling_face:
https://github.com/godotengine/godot-demo-projects/tree/master/gui/theming_override

To do this, a user has to figure out that they need to use <control>.set("custom_constants/...", <value>)

That approach relies on a magic setter. The safer/more "proper" approach is to use add_*_override("name", value) (where * is color, constant, font, icon, shader or stylebox). (In 4.0, it's add_theme_*_override() instead.)

Seriously, what is the type argument in theme.set_constant(...) supposed to be?

type is the node class you want to apply the theme property to (as a string). For instance:

theme.set_constant("line_spacing", "Label", 5)

[Error7Studios]

The safer/more "proper" approach is to use add_*_override("name", value)

Thanks! I see it now. I looked in the Label and Theme class, but forgot to check the Control class.

If See: 'add_*_override()' methods in Control class was at the top of the Theme Properties section for the Label/Button/etc., that would be more user friendly.

(or See: 'add_theme_*_override()' methods in Control class for 4.0)

(In 4.0, it's add_theme_*_override() instead.)

Nice.

type is the node class you want to apply the theme property to (as a string)

I see, thanks! That really needs to be explained in the docs somewhere, though.
If I see type I'm going to think of the enums TYPE_STRING, TYPE_ARRAY, TYPE_DICTIONARY, etc.
(And/or perhaps that parameter should be renamed to node_class or node_type or something.)