Yeah, the hack that was posted here seems to have been broken with Tiger. However, it is still achievable through ShapeShifter. I modified the SS theme "One," to colorize my sidebar blue.
It takes a couple of ShapeShifter's free programs, and a quick scan through the theme to do so, but it can be done in a few minutes. The problem is that it only applies to ShapeShifter, which is a controversial program in the first place. What with the potential for system instability and all that...
Anyway, I believe you need Unsanity's
GUIkitty to unpack the theme. And then use
ThemePark to edit and repackage the new theme.
EDIT: Wanted to give a bit more info on this, as I finally got around to reconstructing the steps for myself. If you want to go the ShapeShifter route, which is about the only solution I know of for Tiger, download the aforementioned applications.
Open the theme you wish to modify in GUIkitty to decompile it and then open the decompiled theme in ThemePark; here there are various settings -- look for Global Elements -> Text Color -> Sidebar Background. Note that you can also change the default bgcolor of lists/column views, as well. Repackage it and open with ShapeShifter.