Try replace the original one with the one you downloaded. If the downloaded one is in JPG, name it "base.marble.jpg". Then create a pure black image that's the same dimensions as those of the downloaded marble skin and name that one "base.marble.alpha.jpg"
Since the Awesome Time/Score is secret, you will have to beat it in order to find out what it is. Just practice every day, and be ready to keep pressing the 'Restart' button...A LOT!