Would be good to have more explicit description of what each color was used for

Setting up my theme, I don't have a strong sense of how the colors are used. Much more description would be good.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/HUBS-1023)

The issue names no files, tests, or entry points. Start by locating the theme color definitions and any existing theme documentation, then clarify and document what each color is used for; done means the purpose of every color is explained.

Written by the indexing model from the issue text.