function cssSheet(){
if(navigator.appName.substring(0,9) == "Microsoft"){
  document.styleSheets[1].disabled = true;
  document.styleSheets[2].disabled = true
}
else if(navigator.appName.substring(0,8) == "Netscape"){
  document.styleSheets[0].disabled = true;
  document.styleSheets[2].disabled = true
}
else{
  document.styleSheets[0].disabled = true;
  document.styleSheets[1].disabled = true
}

}