Example flake from readme build fail

[Familex]

Code from readme does not build with an error:

minegrub-theme> Running phase: buildPhase
minegrub-theme> ERROR: The file background_options/1.8  - [Classic Minecraft] does not exist.
error: builder for '/nix/store/73dc06kry19343j3srsnh5mbfbpdjczj-minegrub-theme.drv' failed with exit code 1

I have tried pass in minegrub-theme.background background_options/1.8 - [Classic Minecraft] and background_options/1.8 - [Classic Minecraft].png, but that doesn't work either.

But background_options/Alpha.png works.