I wrote this little thing to help myself learn Redux 🙈. It allows you to design CodeMirror Themes: select light/dark theme, adjust colors, and export the CSS file. It also shows the contrast ratio 🌓 of each color. The exported CSS file only works for CodeMirror. Other syntax highlighters likely have very different code parsing algorithms, but it may still be useful for quick design prototyping.
To build this locally, clone the repo and run yarn install and yarn start.