Provide options for handling ExhibitBuilder themes

[kimisgold]

Use case comes from this forum thread: the user wants to apply a style to the main site theme, but would like to avoid applying this style override to exhibits they do not manage themselves. I can also see use cases for the reverse, where the user wants to use CSS Editor only for a particular exhibit.

[zerocrates]

It's not as easy in Classic as S to have something like this that integrates into the exhibits one by one.

A cheapo answer is making sure Exhibit Builder just has usefully targetable body classes or something similar, and documenting the fact. It doesn't provide the isolation of a full-fledged feature but it does allow targeted styles for just one exhibit.