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That's a total of 10 now (we don't go too crazy with them around here, we want them to be nice). Five dark themes and five light themes. All the light themes go full on light UI as well.
We split them up into two columns, available in your Editor Settings:
Another cool random CodePen thing: your preference of syntax highlighting follows you around CodePen. As in, if you're reading a blog post with code in it, it will be highlighted in your theme of choice.
Also! They are all available to apply to any embed theme.