Go to skin.ini in your skin folder. The slashes "//" are so that the game doesn't read them as game info, you can ignore those unless you write like a description so you won't forget what these do.
//The border colour on slider tracks. This is what you want
SliderBorder: 255,255,255
//Force a slider track to a certain colour. This is the color of the insides of sliders
//SliderTrackOverride: 0,0,0