use "monospace" as the final fallback code font

All the code samples on the home page are in an ugly serifed font for me.

The CSS currently has:
  font-family: Monaco, Bitstream Vera Sans Mono, Lucida Console, Terminal;
which means that unless you have one of exactly that set of fonts installed,
you get browser default.

The fix is to add "monospace" to the end of the list.

Author: evmar

_sass/_main.scss

@@ -1117,7 +1117,7 @@ table tr th {
 }
 code,
 pre {
-  font-family: Monaco, Bitstream Vera Sans Mono, Lucida Console, Terminal;
+  font-family: Monaco, Bitstream Vera Sans Mono, Lucida Console, Terminal, monospace;
   color: #333;
   font-size: 12px;
 }