Customize Themes: Clarify active and previewed themes

[celloexpressions]

The current active (live) theme is displayed first in the installed themes list, but not necessarily with anything indicating that it's active. The current previewed theme (in the current customize session) gets different styling and a "Previewed:" prefix on its name in the installed themes view.

Let's clarify which theme is being previewed and whether it's active by highlighting it in the sidebar.

Follow-up to #37661.

[celloexpressions]

Latest mockup from @melchoyce on #37661.

[melchoyce]

Changing this to a defect because it is kind of a regression — the pre-4.9 live preview for themes does say which theme you're currently previewing.

[melchoyce]

No movement, so punting to 4.9.1.

[obenland]

This will need a patch soon if it wants to make it into 4.9.2.