Diff color enhancements, add line number background (#30670)
1. Bring back the background on line numbers. This feature was lost a
long time ago.
<img width="457" alt="Screenshot 2024-04-24 at 01 36 09"
src="https://github.com/go-gitea/gitea/assets/115237/76a7f5a9-c22a-4c72-9f0a-ebf16a66513e">
<img width="473" alt="Screenshot 2024-04-24 at 01 22 47"
src="https://github.com/go-gitea/gitea/assets/115237/eef06cf2-f1b9-40e3-947d-dd5852ec12a3">
<img width="457" alt="Screenshot 2024-04-24 at 02 13 18"
src="https://github.com/go-gitea/gitea/assets/115237/59e317d4-76a7-468c-8a19-10d88c675cc3">
<img width="459" alt="Screenshot 2024-04-24 at 01 23 21"
src="https://github.com/go-gitea/gitea/assets/115237/f1a46f8d-8846-4d78-a9d7-8b7dc18ac6e4">
2. Expanded lines background is now full-line, including line numbers:
<img width="1303" alt="Screenshot 2024-04-24 at 01 37 12"
src="https://github.com/go-gitea/gitea/assets/115237/271eefe2-0869-424e-93fb-ccd8adc87806">
3. Sort affected colors alphabetically in the CSS
Fixes #14603
