Rework label colors (#24790)

Rework label colors (#24790)

Introduce `--color-label-fg`, `--color-label-bg` and
`--color-label-hover-bg`, decoupling the label styles from other color
variables. I've set the colors so that non-interactive labels like on
tabs are dark-on-light on light theme, which imho looks better than
previous light-on-dark.

In the screenshot below, the leftmost label has hover, the second one
has active.

<img width="786" alt="Screenshot 2023-05-18 at 12 48 26"
src="https://github.com/go-gitea/gitea/assets/115237/d989bb68-504a-4406-b5f6-419ed9609f90">
<img width="789" alt="Screenshot 2023-05-18 at 13 04 07"
src="https://github.com/go-gitea/gitea/assets/115237/689a281a-a2b7-45e8-a5ee-dafb7a35e105">

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Co-authored-by: Giteabot <teabot@gitea.io>