If you have a skin.ini in your skin folder, write this line:
SliderBorder: 0,0,0
0,0,0 being pure black and 255,255,255 being pure white. The values are in the RGB order. If you have orange borders, that means you should have this line around. Change the values to what suits you best.
If you don't have a skin.ini, then frist, this is weird since slider borders must be white without it, and second, create one or take one from another folder, which should already be the case since you have orange borders. If you wanna know how skin.ini works,
check this page. This page includes links to other skinning elements.
Next time,
ask here for skinning questions. Hoping that helps!