Heads up! This blog post hasn't been updated in over 2 years. CodePen is an ever changing place, so if this post references features, you're probably better off checking the docs. Get in touch with support if you have further questions.
If you’re a PRO member, one of the little neat benefits is that you can add custom CSS to your own profile. In your Settings, there is an input to put the URL of a Pen where that CSS is (so you can use CodePen to customize CodePen).
Here’s some excellent examples of what different members have done:
If you aren’t PRO yet, remember you can try it out for free!