I am using self hosted GitLab 17.5.
This how it looks like with darkreader on.
Everything is dark except for the code.

There is a separate settings for making code editor dark.
Using GitLab preferences I can internally change in my hosted GitLab: https://gitlab.local/-/profile/preferences

However, it is not an optimal of course.

1st thing: Without having possibility to do anything on gitlab and check how it is created it's really hard to help.
2nd: As there are many differemt themes available making "INVERT" would break all of them. Setting some INVERT/CSS filter for "None" only is again not doable without possibility to open Browsers DevTools and analyze how it is done in the code.

As they share possibility to set Dark manually I would just suggest to do that in gitlab settings. Fastest solutions are better than no solutions. 😉

How I can help here ?
It is internal website without external access.

I can provide the HTML with CSS, does it help ?
It is possible to run the GitLab with docker image if this helps:

https://hub.docker.com/layers/gitlab/gitlab-ce/17.5.5-ce.0/images/sha256-e2e258d4cd666f6b88e72881cf2818dc4f89ff58cec4e7a4b2d3c60ae6af841a

🚀 Quick Start: Run GitLab with docker run

Create folders for persistent data (so your GitLab data survives restarts):

mkdir -p ~/gitlab/config ~/gitlab/logs ~/gitlab/data

Run GitLab 17.5 in Docker

Replace gitlab.example.com with your real hostname (or localhost for local testing):

docker run -d
--hostname gitlab.example.com
--env GITLAB_OMNIBUS_CONFIG="external_url 'http://gitlab.example.com'"
--publish 80:80
--publish 443:443
--publish 2222:22
--name gitlab
--restart always
--volume ~/gitlab/config:/etc/gitlab
--volume ~/gitlab/logs:/var/log/gitlab
--volume ~/gitlab/data:/var/opt/gitlab
--shm-size 256m
gitlab/gitlab-ce:17.5.0-ce.0

⚙️ What this does

--hostname: Hostname GitLab uses internally (important for correct URLs).

GITLAB_OMNIBUS_CONFIG: sets external_url so GitLab knows where it’s hosted.

-p: maps container ports → host ports (80, 443, and 2222 for SSH).

-v: stores GitLab’s config, logs, and repo data persistently on host.

--shm-size: shared memory size required by some GitLab services.

:17.5.0-ce.0: pins the specific GitLab 17.5 Community Edition image tag.
(You can change it to EE version if you have appropriate license.)

🔎 After Starting
🔑 Find the initial root password

GitLab generates a root password on first start. To view it:

docker exec -it gitlab grep 'Password:' /etc/gitlab/initial_root_password

You’ll use this password to log in as root at:

http://gitlab.example.com/

or

http://localhost/ (if using localhost)

For me it would be easiest to have local copy of the page I could investigate. Last time I do not have much time for my tasks and setting up anything new on my PC is always taking more time than expected.

So if you can share some saved page as complete (all css included). Then I can try to analyze it locally on my side.