Default style is set at vboptions > style & language settings > default style. After they log in, they may go to their user cp > edit options (or go to the bottom left of any page) and select another style. You said "Only users that I've made a member of this group get the non-default theme" and that is not possible with default vbulletin. If a style is marked as Allow User Selection > Yes, then any usergroup may select it. I was saying that if you have something that makes it so only one usergroup may use it, then that must be a modification.
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