Move Nix files into a nix directory

Move everything from `config` to the root level.

nix/lib/theme/default.nix

@@ -0,0 +1,12 @@
+{ pkgs, ... }:
+{
+  fonts = {
+    monospace = {
+      name = "MesloLGSDZ Nerd Font Mono";
+      # fc-list : family | sort | grep "Nerd Font"
+
+      package = pkgs.nerdfonts.override { fonts = [ "Meslo" ]; };
+      size = "10";
+    };
+  };
+}