They didn't, though. Making a small change (such as making Pigstep look like the rest) is a common idea, you can't claim ownership of it. Unless they went to your profile, looked at your "normal Pigstep" pack, and though, "You know, I'll do the exact same thing," it isn't stealing your work. Heck, when I first saw Pigstep, making it normal was one of my first thoughts (though now I like the color).