Use theme families to refine user themes (#19936)

This PR changes the way we load user themes into the ThemeRegistry. 

Rather than directly pass a theme family's themes to
`insert_user_themes`, instead we use the new `refine_theme_family ` and
`ThemeFamily::refine_theme`.

This PR should have net zero change to themes today, but sets up
enabling theme variables. We need to do it this way so each theme has
access to it's family when it is refined.

Release Notes:

- N/A