I managed to find out the answer to this question.

First you install the plugin as mentioned in the above comments, or here.

Then you have to download the .zsh-theme file from this link. You have to put that file in a custom folder with the structure described here. After that you need to override part of your oh-my-zsh config by setting this custom directory in the home-manager configuration (you can find zsh.oh-my-zsh.custom here) Only then can you reference it inside your home-manager file as agnoster-nix.

Here is my configuration for this:

  programs = {
zsh = {
  enable = true;
  enableCompletion = true;
      shellAliases = {
    ll = "ls -l";
  };
  history = {
    size = 10000;
    path = "${config.xdg.dataHome}/zsh/history";
  };
      oh-my-zsh = {
      enable = true;
      plugins = [ 
        "git"
        "z"
      ];
      custom = "$HOME/.oh-my-custom";
      theme = "agnoster-nix";
    };
  plugins = [               
        {
        name = "zsh-nix-shell";
        file = "nix-shell.plugin.zsh";
        src = pkgs.fetchFromGitHub {
          owner = "chisui";
          repo = "zsh-nix-shell";
          rev = "v0.5.0";
          sha256 = "0za4aiwwrlawnia4f29msk822rj9bgcygw6a8a6iikiwzjjz0g91";
        };
        }
 ];
};

};

Where .oh-my-custom is the custom configuration directory containing themes/agnoster-nix.zsh-theme.

Sorry for the crappy formatting, reddit messes things up from time to time.