Merge branch 'dark-mode-design-tokens' into 'master'

Replace duplicate SCSS variables with design tokens from gitlab-ui

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/135535



Merged-by: Simon Knox <simon@gitlab.com>
Approved-by: Simon Knox <simon@gitlab.com>
Approved-by: Stanislav Lashmanov <slashmanov@gitlab.com>
Co-authored-by: sdejonge <sdejonge@gitlab.com>