To be clear, It’s disabled in that you can’t create the ImageCache.png or individual PNG’s for each component.
But current Audacity can load existing PNG theme files by adding:
[Theme]
LoadAtStart=1
to audacity.cfg.
There is a “Vista” theme at http://jcsu.jesus.cam.ac.uk/~hdc21/design/audacity/ that might be something you could work with or modify.
But as Steve suggested, some method of easily modifying Audacity themes will be coming back within the next few Audacity releases.
Gale