Change the color selector to save in oklch

Problem/Motivation

I initially wanted to store and use oklab colorspace by default in CSS and in properties, but the math was hard, and the color element did not support saving values other than in RGB.

It really should save the values in that colorspace instead.

Proposed resolution

Refactor the theme settings and figure out how to do this in a color selector. If the theme settings are saved in a different colorspace, then convert them.

Remaining tasks

User interface changes

Color selector changes

API changes

CSS properties

Data model changes

CSS and CSS Properties change.