While on Light Theme, it is very easy to see the text;
…it can become very difficult on Dark Theme, depending on your monitor settings:
On dark theme, I have to get uncomfortably close to my monitor to even see it.
Expected behavior
I expect the map to show colors grey (or any less bright white color) to blue, rather than white to blue when Dark Theme is selected.
Another solution would be adding a stroke around the text with a contrasting color. (Such as a black stroke around a white text)
That would very likely solve the visual issue.