[REVIEW ONLY] Simple Themes usability tweaks

Main improvements:

* Message in Themes dialog: "To get more themes, use Extension Manager."  (_not_ clickable at this point -- see discussion in #8946)
* Message in header of Themes tab in Extension Manager: "To switch between installed themes, use View > Themes..."
* **TODO** @MiguelCastillo: Installing a theme automatically sets it as the current theme

I threw in two smaller tweaks as well:

* Link <label>s in Themes dialog to their corresponding controls (they already had a hand cursor thanks to Bootstrap, but clicking them did nothing)
* Eliminate big blank space at bottom of Themes dialog