split nixos from darwin

required because they don't share all attributes

nixos/applications/calibre.nix

@@ -0,0 +1,19 @@
+{ config, pkgs, lib, ... }: {
+
+  options = {
+    calibre = {
+      enable = lib.mkEnableOption {
+        description = "Enable Calibre.";
+        default = false;
+      };
+    };
+  };
+
+  config = lib.mkIf (config.gui.enable && config.calibre.enable) {
+    home-manager.users.${config.user} = {
+      home.packages = with pkgs; [ calibre ];
+      # home.sessionVariables = { CALIBRE_USE_DARK_PALETTE = 1; };
+    };
+    environment.sessionVariables = { CALIBRE_USE_DARK_PALETTE = "1"; };
+  };
+}