Hmmm... I might suggest just using a plain string and rethinking it later. I don't think that something like this should be toggled arbitrarily because people do not want to pass a theme parameter in their DOM. The only time you WOULD is if you want a specific instance to look a little different, but in that scenario it SHOULD be something they're capable of doing with the transclusion.

### TL;DR:
I recommend dumbing down our code and just making this a string (can be based off uiSelectConfig)