I just almost finished that script. (OK, most of the work was really in extracting the separate images from the original "default" theme... the script is as trivial as expected.)

One question: I'd rather have a three-state icon scheme; is that plausible?
I'm thinking blah-blah-[1-7].[0-4].[mu].png, with the mute separate, so the volume level can, for example, still be shown with a red slash overlaid when muted.
This means instead of 7x6=42, we'd have 7x5x2=70 images per theme, but the redundant ones in themes not maintaining a distinction can be links instead of files, so it needn't waste disk space. It's still a sizable conversion though, and I'm not sure if that's welcome at this point.

If such a change does happen, I strongly recommend we go to either 0- or 1-based for both volume and brightness while we're at it... whose idea was that, anyway?

I'll probably post scripts for both styles, anyway, but wondered if the 3-state way will have been a waste of time.