Oh yes, it's a bug in GBS that I noticed but forgot to report. It seems that variable-width fonts expect a white background. Something must be hardcoded somewhere in GBS code.
The best way forward is to open a new issue in https://github.com/chrismaltby/gb-studio/issues.
That's the reason why I didn't create black background fonts as part of this font pack. I changed the palette to recreate this effect, which won't work in your case (as I assume your game is for DMG only).
