No easy way, I'm afraid.

As I understand it, theymos disabled all but the default theme both as a security-related decision (to reduce the amount of attack surface), and as a maintenance-related decision (to reduce the number of files that need to be touched when fixing bugs and implementing new features). Given SMF's code architecture, I would have done exactly the same thing. It's hard to explain to the uninitiated, but, SMF's conception and implementation of "theming" is very backwards (to my way of thinking).

If I were designing things from scratch, I'd probably use many of the ideas I bumped into back when I was doing a lot of C++ GUI programming (in Qt, wxWidgets, and the like). Qt, as a toolkit, was really nice back in the day and contained a lot of good ideas (back when it was still being developed by a very small and very skilled team with a very clear vision; I'd say that that era started to end for them around the transition from Qt 3 to Qt 4, and things really started turning to mush after Nokia acquired Trolltech).

Anyway, long story short: I don't see a sensible (to me) way to do a theme/style system on SMF (as in, one that wouldn't involve a serious restructuring of the code). I forget who suggested it (maybe Cyrus), but, I guess a carefully-designed feature to let the user replace/extend the forum's CSS could be implemented (though, I have more than a few reservations about doing that, including that the markup isn't really optimized for it, and I've noticed that most people aren't nearly as concerned as I am about running browser extensions, so, they may as well then just install a style-management or user-scripting extension and adjust the forum's frontend that way).

I can't recall where my thinking trailed off the last time I considered this, but, I reckon it was with a hunch that 90% of the problem (from the perspective of users) could be solved by implementing a palette system rather than a full-blown theme system (as in, in Qt's terminology, thinking in terms of QPalette rather than QStyle). That's a much easier lift, and would manifest as a new section under "Look and Layout Preferences" where you could individually pick/adjust semantically-named color entries (and choose between a few completed presets to use either as-is or as a starting point for tweaking). Unfortunately, even something like a simplified palette system would be pretty awkward to successfully retrofit onto the forum's version of SMF (which is already near breaking point, and is a quagmire of sloppy code, half-developed ideas, abandoned disciplines, and difficult-to-reason-about cross-cutting concerns/effects).

This PM from last year still captures most of my position on working within the constraints set by SMF: