Add background highlight to currently active theme in admin

Instead of a small bit of italic text, mark out the current theme
in use with an alert DIV. Clearer, quick to see if you have many
themes to choose from.

Tidy up the if-else block uses as the contents are mostly different
between inactive choices and the active one.

"Use this theme" sounded more direct and action packed than
"Choose this theme" for the button.

Cleans up language file definitions orphaned by removal of
references to Typogarden and installing themes in
commit 82c73cd62c3fc616cb44660d9ac6b4dfbe53ddb9
More clearly mark active theme [merges 'randomecho/theme-selections']
Clean up theme choosing template

- Use well rather than alert to create a colored background. This improves
  the layout inside the diff
- Use nice heading levels inside theme descriptions
- Always use the same heading level for the theme name to avoid jarring
  changes