I've tried a lot and I don't think that it's possible. I couldn't ever get it no matter what I tried. It made me mad because on my skin I had transparent hit circles but not sliders.
Should divert all future questions to here; t/51694
Also no, you cannot change slider opacity.
might be able to in the future though t/103885/ anyways, at this current point, can't be done.you can make the slider body black and the dim the background 100%. but thats the best your gonna get for now. check out MLGnom's thread for info on how to do that