With #GitLab 17.0 (stated to be released on May 16th), the operating system theme preference is now respected, as there is now an opt-in setting to enable "Auto (Experiment)" support in your user preferences panel.
It can finally switch between light & #darkmode when your OS or desktop environment requests it at various times of the day! I tried it; it works.
You can find it under "User Settings > Preferences > Appearance" when running that GitLab version (it is already the case on gitlab.com)
