How to edit an existing theme?

How to edit an existing theme?

There does not seem to be any way to edit a theme once I have created it. It would be very useful to be able to save a theme as a text file e.g.


fwDefault=normal
fsDefault=1em
cornerRadius=3px
bgColorHeader=cccccc
...
and to be able to modify this and regenerate the CSS.

This would enable be to iterate the theme design.

As it is, at best I can do is to save the URL (which is just some hex sequence).