home: wm: i3: use grey background

Because I sometimes do get a glimpse of the background, and would rather
not have anything on it.

modules/home/wm/i3/default.nix

@@ -387,5 +387,10 @@ in
         };
       };
     };
+
+    # Use a grey background
+    xsession.profileExtra = ''
+      ${lib.getExe pkgs.xsetroot} -solid '#333333'
+    '';
   };
 }