Update UI look on macOS

macOS no longer uses the old Aqua look, so tweak our things to fit
in better with the current state of things.
diff --git a/vncviewer/vncviewer.cxx b/vncviewer/vncviewer.cxx
index 53ff4b0..426be27 100644
--- a/vncviewer/vncviewer.cxx
+++ b/vncviewer/vncviewer.cxx
@@ -185,14 +185,14 @@
   // Basic text size (10pt @ 96 dpi => 13px)
   FL_NORMAL_SIZE = 13;
 
-#ifndef __APPLE__
   // Select a FLTK scheme and background color that looks somewhat
-  // close to modern Linux and Windows.
+  // close to modern systems
   Fl::scheme("gtk+");
   Fl::background(220, 220, 220);
-#else
-  // On Mac OS X there is another scheme that fits better though.
-  Fl::scheme("plastic");
+
+  // macOS has a slightly brighter default background though
+#ifdef __APPLE__
+  Fl::background(240, 240, 240);
 #endif
 
   // Proper Gnome Shell integration requires that we set a sensible