docs(design): color and theming docs update

[xelaint]

PR Checklist

Please check if your PR fulfills the following requirements:

• [x] The commit message follows our guidelines: https://github.com/graycoreio/daffodil/blob/develop/CONTRIBUTING.md#commit
• [ ] Tests for the changes have been added (for bug fixes / features)
• [ ] Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[x] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

Fixes: #2901

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[xelaint]

@gracetxgao This is still WIP, but can you take a look at what I've currently updated and lmk there are any changes you'd make to the theming guide?