Dark mode is broken in MediaWiki 1.43

Vector 2022 now has its own dark mode called night mode (see e.g. https://wiki.archlinux.org/title/Installation_guide?vectornightmode=1) and our arch_common.less and vector.less need fixing.

Each background, background-color, color, etc. needs to be evaluated and defined properly.

/cc @archlinux/teams/wiki/admins @pierre