Wrappers vs. Dotfiles

Usually user applications (like editors, etc.) get configured through dotfiles in the user's home directory. An alternative, declarative approach is to create wrappers for application on a per-user basis, like this:

{
  users.users.root.packages = [
    (pkgs.writeScriptBin "htop" ''
      #! ${pkgs.bash}/bin/bash
      export HTOPRC=${pkgs.writeText "htoprc" ...}
      exec ${pkgs.htop}/bin/htop "$@"
    '')
  ];
}

The disadvantage of this way is that it doesn't propagate man pages and other paths from the old derivation. Please refer to Nix_Cookbook#Wrapping_packages to possible solutions to retain all outputs.

You can use this simple function which takes care of wrapping the script & symlinking

 writeShellScriptBinAndSymlink = name: text: super.symlinkJoin {
    name = name;
    paths = [
      super."${name}"
      (super.writeShellScriptBin name text)
    ];
  };

Downside of the Wrapper Approach

There might be applications that don't provide means to specify configuration. One could override $HOME, but then there might be applications that require $HOME for other stuff than configuration.
Applications cannot write their configuration anymore, e.g. htop will just terminate without error and nothing changed.

Alternatives

Home Manager manages dotfiles in the user's home directory