That’s a good idea, but I don’t know if is really feasible.
The knob is an image and should be regenerated with a transparent stripe in order to color with css and from what I see there are several knobs (3 to 17 steps and more).
Here a quick hack: dirty edited the knob.png file to make the stripe transparent and added some css class and divs to paint the stripe.
Looks good, but not perfect:
or this with a thinner top border:
Andrea