Dot has a SVG output mode that's reasonably easy to tweak/manipulate. I did an XSL file that adds gradients and drop-shadows to the Dot output back in the day [1] and there are a number of annoying corner cases to deal with, but it's not too horrible; would've been easier in something other than XSL though.
Very nice! I've used little .ps icon files before, and been generally unhappy with the whole thing.
At its heart, graphviz is great. But the tools so need a nice little modernizing.
At the basic level, I found myself being very repetitive because its not possible for graphviz nodes to inherit attributes from others etc. No CSS kind of hierarchy.
[1] https://github.com/vidarh/diagram-tools