I would imagine you would do it using CSS.
However, thanks for providing the trigger for finding the answer to my unanswered question about how to set default themes!
https://forum.jquery.com/topic/please-let-us-set-default-swatch-assignmentsI started searching the source code, and then with a bit of Googling confirmed what was emerging as the answer. It's explained pretty well here:
http://stackoverflow.com/questions/7924665/change-the-default-data-theme-from-jquery-mobileThe literal "a", "b", "c" constants sprinkled all over the source is a horrible programming practice. I don't know what the developers were thinking.
I'd love to at least see these prototype options documented! There is a bit in the documentation on forms.
However, looking at the source code, there are some of those literal constants elsewhere where they would not be so easy (or even possible) to patch like this.