It would be a great feature one day, but today......
Themeroller is unidirectional, but only on a good day, many days it's anti-directional :<
They've done an amazing job designing themeroller so that it's unidirectional nature isn't so bad. You can easily remember Theme elements visually (i.e. three o'clock on the color wheel, upper right quadrant for lightness/color saturation). Besides, many times a theme only takes a handfull of distinct color settings, and completing a theme is mostly copy/paste of the background/icon/text/ colors between the various states. I was at first distressed to realize Theme roller is unidirectional, but it aint that bad.
At some point, you may also decide to just start editing theme css by hand, also doable if your theme is very complex with lots of color choices and variations, and doing over is not an option.