Add a FORCE_THEME environment variable

[mlmiltcheva]

Hi there!

I'm proposing a FORCE_THEME environment variable that will set the theme for the project (light or dark), and remove the user's ability to change the theme. If unset (default), everything functions as is. The reasoning is that I'd like to have two separate instances of shelfmark with different purposes. While changing the Search Page Title is helpful to differentiate the two instances, I'd like for something that stands out a bit more.

Thanks!