Setting up BigSur grub theme ( problem fetching background.png )

Help
1

I decided to fetch a Grub-theme from github and specify the directory where there is the theme.txt.

I was inspired by this post. This is my current implementation:

    boot.load.grub = {
            device = "nodev";
            efiSupport = true;
            useOSProber = true;
            splashImage = null;
            theme = "${
                (pkgs.fetchFromGitHub {
                  owner = "Teraskull";
                  repo = "bigsur-grub2-theme";
                  rev = "2e8139afd2b6e4cb2dcaf85d8248cbc10d556c6b";
                  sha256 = "71e253SUEMBTt1m5Hq+sinvuT6KSLt44QNEWxGp4Uc0=";
                })
            }/bigsur/";
        };
    };

The problem is the background.png seems to give an error saying:

error: bitmap file `(hd0,gpt1)//theme/background.png` is of unsupported format.
Press any key to continue...

I have used this theme before (not in nixos ), and I can assure it works. I will provide any hardware info on request

This is the theme github: GitHub - Teraskull/bigsur-grub2-theme: A Grub2 Bootloader Theme, compatible with Ventoy.