Merge branch '425313-fix-scroll-scrim-bg-color' into 'master'

Fix background color of sidebar scroll scrim

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/138156



Merged-by: Lukas 'Eipi' Eipert <leipert@gitlab.com>
Approved-by: Lukas 'Eipi' Eipert <leipert@gitlab.com>
Approved-by: Austin Regnery <aregnery@gitlab.com>
Reviewed-by: Austin Regnery <aregnery@gitlab.com>
Co-authored-by: Thomas Hutterer <thutterer@gitlab.com>