# Disable UEFI.
  boot.loader.systemd-boot.enable = false;

  # Use the GRUB 2 boot loader.
  boot.loader.grub.enable = true;
  boot.loader.grub.efiSupport = false;

  boot.loader.efi.canTouchEfiVariables = false;
  # boot.loader.grub.efiInstallAsRemovable = true;
  # boot.loader.efi.efiSysMountPoint = "/boot/efi";
  # Define on which hard drive you want to install Grub.
  boot.loader.grub.device = "/dev/sda"; # or "nodev" for efi only

{
  inputs.nixpkgs.url = "github:nixos/nixpkgs/nixos-24.11";
  inputs.disko.url = "github:nix-community/disko/latest";
  inputs.disko.inputs.nixpkgs.follows = "nixpkgs";

  outputs = {self, disko, nixpkgs }: {
    nixosConfigurations.koolthing = nixpkgs.legacyPackages.x86_64-linux.nixos [
      ./configuration.nix
      disko.nixosModules.disko
      {
        disko.devices = {
	  disk = {
          main = {
            type = "disk";
            device = "/dev/vdb";
            content = {
              type = "gpt";
              partitions = {
                # Legacy BIOS, MBR-style partition table.
                boot = {
                  size = "1M";
                  type = "EF02"; # Grub MBR.
                };

                # The LUKS-encrypted root.
                luks = {
                  size = "300G";
                  content = {
                    type = "luks";
                    name = "crypted";

                    # Set up with an interactive password and not a keyfile.
                    settings = {
                      allowDiscards = true;
                    };

                    # The Btrfs filesystem.
                    content = {
                      type = "btrfs";
                      extraArgs = ["-f"]; # Overwrite any existing file system.
                      subvolumes = {
                        "/root" = {
                          mountpoint = "/";
                          mountOptions = [
                            "compress=zstd"
                            "noatime"
                          ];
                        };

                        # The home subvolume.
                        "/home" = {
                          mountpoint = "/home";
                          mountOptions = [
                            "compress=zstd"
                            "noatime"
                          ];
                        };

                        # The nix subvolume.
                        "/nix" = {
                          mountpoint = "/nix";
                          mountOptions = [
                            "compress=zstd"
                            "noatime"
                          ];
                        };
                      };
                    };
                  };
                };
              };
            };
          };
	};
        };
      }
    ];
  };
}

  cssBase = builtins.readFile (
    pkgs.replaceVars ./waybar-style.css {
      base03 = "#002b36";
      doesntExist = "#073642";
    }
  );

nix error

  error: builder for '/nix/store/ljzv9k9dj9n6hh3mgsxwjw161qcw6a40-waybar-style.css.drv' failed with exit code 1;
       last 6 log lines:
       > Running phase: patchPhase
       > Running phase: updateAutotoolsGnuConfigScriptsPhase
       > Running phase: configurePhase
       > no configure script, doing nothing
       > Running phase: buildPhase
       > substituteStream() in derivation waybar-style.css: ERROR: pattern @doesntExist@ doesn't match anything in file '/nix/store/x6b14c7gkk03wkh1pgsw2klffzv4hdsf-waybar-style.css'
       For full logs, run:
         nix log /nix/store/ljzv9k9dj9n6hh3mgsxwjw161qcw6a40-waybar-style.css.drv 

replaceVars doesn't ignore extras in the set. Is there a way to ignore the extras?

That's intentional; the idea is that you only pass the variables you actually want to replace to replaceVars.

That's one of the main reasons why replaceVars is preferred over substituteAll.

If you don't know what vars will be available to replace, you should continue using substituteAll. However generally it's better to be explicit and only pass the variables you actually want to replace to replaceVars.

let
  colours = {
    # ...
  };
  cssBase = builtins.readFile (
    pkgs.replaceVars ./waybar-style.css {
      inherit (colours)
        base03
        ;
    }
  );
in

Side-note: passing your replaceVars derivation to builtins.readFile is an example of Import From Derivation. This is often a bad idea; it leads to poor performance; it also isn't allowed in the nixpkgs repo.

That's intentional; the idea is that you only pass the variables you actually want to replace to replaceVars.

In most cases, the input variables not matching the variables present in the template file is a bug; e.g. a spelling error; or a change in the template file that hasn't been updated in the input variables.

That's one of the main reasons why replaceVars is preferred over substituteAll.

If you don't know what vars will be available to replace, you should continue using substituteAll. However generally it's better to be explicit and only pass the variables you actually want to replace to replaceVars.

let
  colours = {
    # ...
  };
  cssBase = builtins.readFile (
    pkgs.replaceVars ./waybar-style.css {
      inherit (colours)
        base03
        ;
    }
  );
in

Side-note: passing your replaceVars derivation to builtins.readFile is an example of Import From Derivation. This is often a bad idea; it leads to poor performance; it also isn't allowed in the nixpkgs repo.