bash: No such file or directory during a grub theme install

[Isukiipko]

So im trying to install the big sur grub theme and im following the install guide:

$ git clone https://github.com/Teraskull/bigsur-grub2-theme

$ cd bigsur-grub2-theme

$ ./install.sh

and for some reason when inputting $ ./install.sh i get
bash: ./install.sh: cannot execute: required file not found

and i dont really know what to do... (Btw im using NixOs with the grub bootloader and i also have windows 10 installed with a dual boot also heres my nix config file)