confiuration.nix

...
 environment.systemPackages = with pkgs; [
    (pkgs.oh-my-zsh.overrideAttrs (oldAttrs: rec {
    phases = "installPhase customThemePhase";
    srcTheme = fetchurl {
        url = "https://raw.githubusercontent.com/NWuensche/dotFiles/master/terminalStuff/agnoster.zsh-theme";
        sha256 = "b99455c561bdcf9ec0601669da3d1aa680328ec1836430de22c6e7e32497ea5b";
    };
    customThemePhase = "sudo cp $srcTheme $outdir/themes/agnoster.zsh-theme";
  }))
...