Apparently that's "legible" and "looks the same in all... browsers".
I certainly agree that BMFW has less contrast than the MFW; so little that it's difficult to read!
For comparison, here's how MFW looks: https://imagebin.ca/v/2eeVa5QksF6e
White text on a black background, exactly as I asked for when I configured Firefox. Content taking up my whole widescreen monitor, exactly as I asked for when I made the Firefox window that size.
The only thing BMFW seems to have correct is using sans-serif font; which I expect is because I unticked Firefox's "allow pages to override these fonts" option. Other than that, it looks like a me-too cargo-cult of MFW which completely misses the point.
Presumably the creators of BMFW are using, and only ever test anything with, a black-on-white style which is, let me guess, the browser's default? What would that make them, in their own (jokingly "quoted") words?