You've got two options for customzing your profile on CodePen, and endless possibility:

Open to everyone: Apply Custom CSS (which can be a link to a Pen on CodePen)PRO only: Apply a Pen-as-Header-Background

In this video, Chris & Stephen look at a bunch of awesome profiles on CodePen, and find that many users (especially the coolest profiles) do both. Here's the shuffle machine we used in the video to randomize cool profiles: