I'm a huge fan of CodePen (No, they didn’t pay me to write this). I'm using it for prototyping, experimenting, sharing code, and in my latest blog post, The Dark Side of the Grid, I'm also making use of their Embedded Pens.
CodePen allows you to customize syntax highlighting, and background and text colors of UI elements in Embedded Pens.
As a PRO user, I can also add custom CSS, which gives me the ability to improve Pens not just visually but in terms of accessibility.