Where are the PNG files for the UI theme stored? I like using my Mac with the "graphite" theme, but I'd like to still keep the color "traffic light" graphics.  I remember from various theme modifiers in the past that these are simple PNG files, so I figured if I copied the color ones into the graphite directory it would use them.

I forget where in the System these files are.  Anyone know?
You might try

/System/Library/PrivateFrameworks/CoreUI.framework/Versions/A/Resources/UI.bundle/Contents/Resources