Gitlab wiki - Diagrams.net diagram renders as a negative

I'm using the "insert or edit diagram" functionality, from the plain text editing view, in Gitlab wiki:

I craeted a test diagram, and it shows up in the preview mode and when saving the page, but in both cases it is rendered as a negative:

I'm using the Light theme, and Gitlab is a self-hosted instance running version 17.11. I checked this and this gitlab documentation pages, but I couldn't find any info. I also played with diangrams.net while creating the diagram to try to find some relevant setting, but cannot find it.

Other things that I tried:

• changed the background color, but again it was displayed as negative,
• switched to gitlab dark theme and then again to light theme.

Is there some configuration, in Gitlab or diagrams.net to change to correctly display the diagram?

The behaviour was due to the theme used in the browser (Firefox): I was using default theme (and the system theme is dark), so that it was actually using dark theme, even if Gitlab is set to use light theme. Selecting light theme also for the browser, the diagram displays with the correct colors.