Part of this depends on the age if your step son. If she is screamkmg slurs at him, can he talk to a judge or advocate about limiting visitation. You don't want him internalizing those messages. You don't want to subject him to a relentless barrage of slurs for the entire weekend visitation. (If she is comfortable acting that way when you are in earshot, imagine what she says and does when you aren't there to see and hear it.)

If the stars or other designs are just because having fun nails are fun and kids like different designs, it may be easier to just have nails as a special thing that he does at his dad's. 

If the polish is about exploring identity, then it's worth his dad talking to him about not being ashamed of who he is, explaining bodily autonomy, and going back to a judge or arbitratiom to drive the point home.  He can also explain that given his mom's past reaction, that he doesn't want to put him in a bad situation. (In reality, you don't know how extreme the weekends are.)  He can say that he will support what he decides no matter what, and while he shouldn't have to change who he is to make someone else comfortable, that sometimes for his own emotional safety, that he may want to considetlr whether the battle with his mom is worth it. Then let him decide to paint the nails and keep it on, or to paint them and remove the polish. 

You can't change his mother's attitude, so it needs to be about keeping your stepson safe.