Yeah, it's been done. Doesn't mean it can't be done again. I love it. It looks best on black, in my opinion, and I think you should keep the gray. Makes it stand out a bit more so it isn't swallowed up by the black t-shirt, and still looks good with that shadow on the other colors.
The Jolly Roger is a recurring theme, yes. Doesn't mean you can't take someone else's idea and improve on it. It's cute, unisex, and surprisingly upbeat for a skull and crossbones. It's not morbid at all, which is wonderful. Nicely done. I'd buy it.