Fix readability issue with CodeMirror on dark mode

[adam-buckley]

In light mode the CodeMirror editor looks fine:

But in dark mode it's not great:

The sidebar is not in dark mode and the constrast of the red colouring makes it very hard to read.

Since dark mode support is kind of there from codemirror, we should be making a CodeMirror.scss file in the new designs and adding our theming overrides there