Set font family on root element

This is a very global setting, so let's put it on the top node for
clarity.

app/styles/base.css

@@ -31,10 +31,13 @@
  * noVNC_disconnecting: Disconnecting from server
  */
 
+:root {
+  font-family: Helvetica;
+}
+
 body {
   margin:0;
   padding:0;
-  font-family: Helvetica;
   /*Background image with light grey curve.*/
   background-color:#494949;
   background-repeat:no-repeat;