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While most coders I know prefer light-on-dark, there's certainly a strong contingent for dark-on-light! CodePen has always been a dark site. Not just in it's default syntax highlighting theme (Twilight), but the entire UI.
That was kind of a bummer for people who chose a dark-on-light theme, because, while the code editors were light, the rest of the UI around them was still dark. It didn't really achieve the lightness I'm sure people who prefer that look desire. Light themes were a second class citizen, you might say.
Now, when you pick a light them (there are only two at the moment, Classic and Solarized Light), the whole UI (in the Editor only) will go light. Like this:
If you have more ideas for syntax highlighting themes you'd like to have available, we have an open thread for that.