how to change scroll box "scroll' images?

Hi, on widgets im using a scroll box and its working fine, but for some reason i am not figuring out how to change that white bar by anything else(doesnt look like even changing colors on all the vertical/horizontal ones seem to do anything)
ideally id be able to change it by an image of mine and if possible even have a different image for the top and down(so i can put arrows).

i wasnt able to find anything online about this so if anyone can offer some input much appreciated.