Create TS Types for themes

[grgia]

Let's create a Color type to ensure that all colors used are from our brand colors.
Let's also create a Theme type to ensure that both themes are the same shape.

[grgia]

@chrispader

[chrispader]

@grgia Working on it here: #24178

[melvin-bot]

This issue has not been updated in over 15 days. @grgia eroding to Monthly issue.

P.S. Is everyone reading this sure this is really a near-term priority? Be brave: if you disagree, go ahead and close it out. If someone disagrees, they'll reopen it, and if they don't: one less thing to do!

[chrispader]

Still on HOLD for #22986