Documentation for new theming & NOAA branding

[theangchen]

Add documentation on how the new theming works

• Add that we are shipping with NOAA branding as default. Add a screenshot of what it looks like
• Explain how the new theming structure works & how to make changes if they need to or add their own theme.

We can't ship without updating docs.
(Targeting RC for branding week of Feb 15)