Import a Themeroller-based package back to themeroller for further tweaking

I have a jQueryUI theme built by Themeroller (1.7.2) that i'd like to tweak via Themeroller. I recall seeing there was a method to use an existing custom theme as a starting point for a new theme - essentially importing the old theme so additional tweaks could be executed.

Can't see instructions on how to do that now.