De-emphasize issue sidebar buttons (#26171)

De-emphasize issue sidebar buttons (#26171)

I find the colored buttons in the issue sidebar distracting, given that
they are not primary actions, I think we can de-colorize them.

Before:
<img width="285" alt="Screenshot 2023-07-26 at 19 42 22"
src="https://github.com/go-gitea/gitea/assets/115237/7e784805-4e01-4199-94bb-0538a0130264">
<img width="288" alt="Screenshot 2023-07-26 at 19 43 06"
src="https://github.com/go-gitea/gitea/assets/115237/3a89c661-e24a-4ebf-a585-d404d0a6a78a">
<img width="285" alt="Screenshot 2023-07-26 at 19 44 36"
src="https://github.com/go-gitea/gitea/assets/115237/c1aa8c13-6f41-4763-8149-d1c07cb4be5c">:

After:
<img width="286" alt="Screenshot 2023-07-26 at 19 42 04"
src="https://github.com/go-gitea/gitea/assets/115237/74d640c2-e0ab-4fef-87aa-9e788e9010e2">
<img width="285" alt="Screenshot 2023-07-26 at 19 42 51"
src="https://github.com/go-gitea/gitea/assets/115237/3b69976a-9aa4-4e1c-8df3-4168f4a9fcf9">
<img width="286" alt="Screenshot 2023-07-26 at 19 45 15"
src="https://github.com/go-gitea/gitea/assets/115237/897222fd-4df2-4d99-98eb-e5f8fb77c4d6">