Merge branch 'update-startup-css' into 'master'

Update startup CSS stylesheets

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/103675



Merged-by: Mark Florian <mflorian@gitlab.com>
Approved-by: Michał Zając <mzajac@gitlab.com>
Approved-by: Mark Florian <mflorian@gitlab.com>
Co-authored-by: Paul Gascou-Vaillancourt <paul.gascvail@gmail.com>