Markup color and font size fixes (#30282) (#30310)
Backport #30282 by @silverwind
1. Distinguish inline an block code with new CSS variable
`--color-markup-code-inline`
2. Various color tweaks, better contrast from background
<img width="447" alt="Screenshot 2024-04-05 at 00 51 00"
src="https://github.com/go-gitea/gitea/assets/115237/
93e069f4-6807-4f2c-9331-
2d69730919d4">
<img width="456" alt="Screenshot 2024-04-05 at 00 50 44"
src="https://github.com/go-gitea/gitea/assets/115237/
0dc9c745-c531-40fa-94ec-
b0ba10bd7ccf">
Co-authored-by: silverwind <me@silverwind.io>