Dark theme for markdown editor

[btzr-io]

Bug

The current markdown editor needs a better dark theme, right now it only changes the background color of the form field, but not the text colors.

Related issues, but there are probably more:

• [ ] Inconsistent cursor color on dark theme, its hard to see.
• [x] Hyperlinks on markdown hard to read in dark mode #3468

Expected behavior A good contrast of colors for both themes : light / dark

[btzr-io]

This will be a tedious process, since codemirror classes are mixed with easymde styles