The game either doesn't fill the screen or the fonts get extremely hard to read
How do I fix this

Hi there! Are you using Windows, Mac, or Linux? What happens when you try to make it fullscreen, does it stay in a window or does it indeed go fullscreen just with black bars? And what is your screen resolution?

We specifically made it so the game doesn't scale to decimal values, because like you said the pixel fonts get kinda weird at anything but 1x/2x/3x/4x/etc resolutions. If you like, you can try using the in-game BIOS to force "non-integer scaling", but I can't guarantee that will look any good on your display. Alternatively, you can also change the resolution on your monitor - I know that's not the best answer but it might look better than non-integer scaling.