Darkreader does a really good job of preserving visibility in most cases, and is available on both mobile and desktop browsers. Definitely reconnect checking it out.
For me (Firefox on macOS, also tested with Chrome on macoS), the site's dark theme has a purple background for the diagram which makes it readable even though the background is black.
However, the SVG seems to get all its color from CSS; the color is not part of the SVG itself. Maybe that's not working in your browser?