Allow users to toggle editor theme

[mrjoelkemp]

Would be nice to have some personalization built in without having to fork the project.

Saving theme choice via localstorage would be great. Ideally, the entire site would adopt the theme. In other words, if you choose a dark theme for the editor, the site should adopt a dark theme so the contrast isn't jarring.

There should be a dropdown in the left sidebar with the possible Code Mirror themes.

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