Build problem when using it through NixOS Flake

[0nyr]

I'm trying to use Minegrub on my NixOS, using a Flake for the whole config. Unfortunately, I get an error while building, related to minegrub:

[root@nixos:/etc/nixos]# nix log /nix/store/wr9q40ndd6wnpiw869vh9j5360g2qlb6-minegrub-theme.drv
[root@nixos:/etc/nixos]# nixos-rebuild build
building the system configuration...
error: builder for '/nix/store/wr9q40ndd6wnpiw869vh9j5360g2qlb6-minegrub-theme.drv' failed with exit code 2;
       last 5 log lines:
> unpacking sources
> unpacking source archive /nix/store/qmlsa71lfnwvlp4cph73k789z9ajg418-source
> source root is source
> patching sources
> sed: can't read update_theme.py: No such file or directory
       For full logs, run 'nix log /nix/store/wr9q40ndd6wnpiw869vh9j5360g2qlb6-minegrub-theme.drv'.
error: 1 dependencies of derivation '/nix/store/i048izr3c7ffzqs2c0789qzlahgpalgf-grub-config.xml.drv' failed to build
error: 1 dependencies of derivation '/nix/store/0h9zdylh13wbjchfwqrk82lg50klv93b-install-grub.sh.drv' failed to build
error: 1 dependencies of derivation '/nix/store/z0agq1y58lwdbmqksxzimjfmys78djb0-nixos-system-nixos-23.11.20230831.3e52e76.drv' failed to build

[root@nixos:/etc/nixos]# nix log /nix/store/wr9q40ndd6wnpiw869vh9j5360g2qlb6-minegrub-theme.drv^*
@nix { "action": "setPhase", "phase": "unpackPhase" }
unpacking sources
unpacking source archive /nix/store/qmlsa71lfnwvlp4cph73k789z9ajg418-source
source root is source
@nix { "action": "setPhase", "phase": "patchPhase" }
patching sources
sed: can't read update_theme.py: No such file or directory

The issue is related to the minegrub-theme package not being built properly. It appears that the build process expects a file named update_theme.py which it can't find. Here is the github of the related project that contains a flake.nix: https://github.com/Lxtharia/minegrub-theme.

Here is my current /etc/nixos/flake.nix:

# flake.nix for /etc/nixos/
{
  description = "Custom NixOS configuration";

  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
    minegrub-theme.url = "github:Lxtharia/minegrub-theme";
  };

  outputs = {nixpkgs, ...} @ inputs: {
    # replace whatever comes after nixosConfigurations with your hostname.
    nixosConfigurations.nixos = nixpkgs.lib.nixosSystem {
      system = "x86_64-linux";
      modules = [
        ./configuration.nix
        inputs.minegrub-theme.nixosModules.default
      ];
    };
  };
}

I'm still a noob with NixOS and Flakes, so the issue can be from the project itself or from my side. Feel free to ask for anything.

Any help appreciated.