I had this problem too and dug a little into things. Had to load the game in firefox so I could use the development console.
The issue is that gamefont.css tries to load 8bitoperatorplus-regular.ttf, while the file is named 8bitOperatorPlus-Regular.ttf. Linux distinguishes filenames with diferent capitalization.
My solution was making a system link because I don't like changing game data, but you can just rename the file safely.