Projects have themes and those are consulted

Finishes out the logic for determining which theme to use in any given run of an activity. Consults first activity, then sequence, then project, then the site default. [#52741671]

https://www.pivotaltracker.com/story/show/52741671
Adding useful text to blank option in theme menus

@ddamelin suggested when theme is optional, we explain what's happening, as the blank option is confusing when a theme named "default" is also an option. This turns out to be pretty easy.