I know this is not the answer you are looking for, but: Don't mess with people's scroll bars.
Messing with scroll bars and natural behavior results in one of the following problems:
Browser compatibility issues
The design will appear to be an eyesore (study after study shows that people "ignore" the scroll bars due to familiarity, changing their look will therefore have the opposite effect, despite your intentions)
If the normal scroll bars don't fit your design ... then I am inclined to believe the form elements are way too over-styled ...