Ran most of the Less files through the Less compiler and Prettier and
then followed up with a round of manual fixes.
The Less compiler had unfortunately stripped all `//` style comments
that I had to restore (It did preserve `/* */` comments). Other fixes
include duplicate selector removal which were revealed after the
transpilation and which weren't caught by stylelint before but now are.
Fixes: https://github.com/go-gitea/gitea/issues/15565
- Extract CodeMirror-related styles to separate files
- Generalize CodeMirror styles where possible
- Improve fullscreen and side-by-side mode for dark theme
Co-authored-by: 6543 <6543@obermui.de>
