[DOCS] `@daffodil/design` Theming Guide

[xelaint]

:page_facing_up: Documentation Request

What were you doing?

I was trying to figure out how to use all the theming features of @daffodil/design

Expected behavior

Add guides on how to use Daffodil's theming features

• [x] How color works
• [ ] How to use daff-illuminate (#1412)
• [ ] How to use daff-text-contrast
• [ ] Theming tokens/variables

Existing Documentation

Environment

Daffodil version: X.Y.Z
Angular version: X.Y.Z 

 
For Tooling issues:
- Node version: XX  
- Platform:  

Others: