Improve feed icons and feed merge text color (#27498) (#27716)
Backport #27498 by @silverwind
1. Improve various feed icons
2. Fix merge message color
<img width="763" alt="image"
src="https://github.com/go-gitea/gitea/assets/115237/
3f5bcb23-6d90-4c63-85f2-
46bd7e1c96d6">
<img width="769" alt="image"
src="https://github.com/go-gitea/gitea/assets/115237/
466c37b4-e2f4-42bb-922d-
b86596cdc6d0">
Fixes: https://github.com/go-gitea/gitea/issues/27495
Continues: https://github.com/go-gitea/gitea/pull/27356
Co-authored-by: silverwind <me@silverwind.io>