Add background highlight to currently active theme in admin

The italic text to show what theme is active is too small. This marks out the current theme in use with an alert DIV. 

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.

This also cleans up some language file definitions orphaned by removal of references to Typogarden and installing themes in commit 82c73cd62c3fc616cb44660d9ac6b4dfbe53ddb9