Improve contrast by lightening the text colors in dark theme by around
35%. Additionally, share some variables that had the same or similar
color, which will ease future theme creation.
Part of https://github.com/go-gitea/gitea/issues/27097:
- `gitea` theme is renamed to `gitea-light`
- `arc-green` theme is renamed to `gitea-dark`
- `auto` theme is renamed to `gitea-auto`
I put both themes in separate CSS files, removing all colors from the
base CSS. Existing users will be migrated to the new theme names. The
dark theme recolor will follow in a separate PR.
## ⚠️ BREAKING ⚠️
1. If there are existing custom themes with the names `gitea-light` or
`gitea-dark`, rename them before this upgrade and update the `theme`
column in the `user` table for each affected user.
2. The theme in `<html>` has moved from `class="theme-name"` to
`data-theme="name"`, existing customizations that depend on should be
updated.
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Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: Giteabot <teabot@gitea.io>
