Merge branch 'super-sidebar-themes' into 'master'

Move super sidebar theme mixin to super sidebar styles

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/139727



Merged-by: Paul Gascou-Vaillancourt <pgascouvaillancourt@gitlab.com>
Approved-by: Ravi Kumar <rkumar@gitlab.com>
Approved-by: Thomas Hutterer <thutterer@gitlab.com>
Approved-by: Paul Gascou-Vaillancourt <pgascouvaillancourt@gitlab.com>
Reviewed-by: Paul Gascou-Vaillancourt <pgascouvaillancourt@gitlab.com>
Reviewed-by: Thomas Hutterer <thutterer@gitlab.com>
Co-authored-by: sdejonge <sdejonge@gitlab.com>