I'm not using any dark theme addons or similar, just the standard prefers-color-scheme, and I have the same problem. So they just didn't account for their website's dark mode when writing the article.
I've just pushed a workaround that appears to have fixed it.
I clicked the light mode on the website and then back to dark mode and it was fixed. So their website is reading your theme and adjusting the background correctly but fails to adjust the test and diagrams.