Changing the code-font font-family to prefer Menlo and Consolas over Monaco,...

Changing the code-font font-family to prefer Menlo and Consolas over Monaco, and adding more back-up fonts.

source/stylesheets/variables.scss

@@ -75,7 +75,7 @@ $h1-margin-bottom: 21px; // padding under the largest header tags
 }
 
 %code-font {
-  font-family: Monaco, "Courier New", monospace;
+  font-family: Consolas, Menlo, Monaco, Lucida Console, Liberation Mono, DejaVu Sans Mono, Bitstream Vera Sans Mono, Courier New, monospace, serif;
   font-size: 12px;
 }