Looks like you need a Unity upgrade, in fullscreen the Linux version doesn't display anything (a well known bug from earlier Unity versions).
Yeah, I'm not sure how to fix that. I'm using Unity 2018.2.14f, which is pretty recent. As I mentioned in the development builds, there's a workaround:
"Linux builds might start with bugged graphics. To solve this, go to ~/.config/unity3d/Flaming Torch games/Fairtravel Battle/, edit the prefs file, and change the fullscreen line to have a value of 0, and remove the two lines that give a width and height of 1."
Sorry for the trouble, though. Whenever I figure out how to properly fix this, I'll do so. It's a critical bug on Linux.