var smallStylesheetLink  = "css/style-800.css";
var normalStylesheetLink = "css/style-1024.css";
var useSmall;

function setStyle() {
   var width, height;
   if(window.innerWidth) {
      width = window.innerWidth;
      height = window.innerHeight;
   }else if(document.documentElement.clientWidth) {
      width = document.documentElement.clientWidth;
      height = document.documentElement.clientHeight;
   }else if(document.body.clientWidth) {
      width = document.body.clientWidth;
      height = document.body.clientHeight;
   }

   var stylesheetLink = document.getElementById("stylesheetLink");
   var useSmall = (width <= 800 && height <= 600);
   stylesheetLink.setAttribute("href", (useSmall ? smallStylesheetLink : normalStylesheetLink));
   
   return useSmall;
}

useSmall = setStyle();