Add a way to pick colors consistently

[stilnat]

Summary

Working on UI means having to use the same colors a lot, if we expect some consistency. We currently define those colors by hand.
We need a centralized script, or scriptable object or whatever, that allow us to easily fetch some colors that we can use in the UI.
This will also be beneficial to try and change one color everywhere, for example, colors of all buttons.

Goal

• Create said central color picker
• Add a bunch of colors already in the current UI
• Document how to access those colors in code, or if possible, even in editor (but not a priority).

[cosmiccoincidence]

Going to work on a design for this sometime soon and probably pick some colors too (they can easily be changed later if needed).

[ziyuxuan84829]

Claiming this bounty. I'll implement the centralized color picker system.