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?
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?