Rather than hijack Mat's thread I made a new one.
I am working on a plugin that would add a simple interface for the many possible tweaks that are possible but not currently easily accessible in tiddlywiki. I have something that works, and I like the interface. At the moment I only have it set up to automatically generate CSS, but other configuration options can be added.
As usual I got a bit distracted and I think that the css generation part may end up being the more interesting thing. I made it so that you can override properties defined in existing classes, but it can be used to create new ones too.
A demo with the usual semi-coherent explanation is
here.