List of overlays to use with the Nix Packages collection. (For details, see the Nixpkgs documentation.) It allows you to override packages globally. This is a function that takes as an argument the original Nixpkgs. The first argument should be used for finding dependencies, and the second should be used for overriding recipes.
If null, then the overlays are taken from the fallback location, for example, ~/.config/nixpkgs/overlays.
Like nixpkgs.config this option only applies within the Home Manager configuration. See nixpkgs.config for a suggested setup that works both internally and externally.