reveal.js template: move custom css after theme.

This allows custom css to modify themes, instead of being replaced
by themes.

Closes #91.

default.revealjs

@@ -20,14 +20,14 @@ $if(highlighting-css)$
 $highlighting-css$
   </style>
 $endif$
-$for(css)$
-  <link rel="stylesheet" href="$css$"/>
-$endfor$
 $if(theme)$
   <link rel="stylesheet" href="$revealjs-url$/css/theme/$theme$.css" id="theme">
 $else$
   <link rel="stylesheet" href="$revealjs-url$/css/theme/black.css" id="theme">
 $endif$
+$for(css)$
+  <link rel="stylesheet" href="$css$"/>
+$endfor$
   <!-- If the query includes 'print-pdf', include the PDF print sheet -->
   <script>
     if( window.location.search.match( /print-pdf/gi ) ) {