- Upgrade stylelint and plugin
- Change ruleset to a explicit one, with all deprecated rules removed
- Fix new issues detected by value validation
For `overflow: overlay` see
https://github.com/stylelint/stylelint/issues/6667
This should eliminate all non-variable color usage in the styles, making
gitea fully themeable via CSS variables. Also, it adds a linter to
enforce variables for colors.
