Yes, the Handle has an appearance setting for its Image. I don’t remember if it can be set like a Button where you have Box, Border or Image type drawing, but you can set the Image, and maybe(?) the Tint on that image which is handy for greyscale.
I have changed the little grey rectangle to a round icon myself. I haven’t tried changing the color yet.
Worst case scenario is you can build your own slider widget but I don’t think you’ll have to do that.