Stamp = new Date();
Hours = Stamp.getHours();

if( (Hours>=0) && (Hours<=5) ) {style_to_use = 'Night';}
if( (Hours>=6) && (Hours<=11) ) {style_to_use = 'Sunrise';}
if( (Hours>=12) && (Hours<=17) ) {style_to_use = 'Day';}
if( (Hours>=18) && (Hours<=23) ) {style_to_use = 'Sunset';}


setActiveStyleSheet(style_to_use);

function setActiveStyleSheet(title) {
   var i, a, main;
   for(i=0; (a = document.getElementsByTagName("link")[i]); i++) {
     if(a.getAttribute("rel").indexOf("style") != -1
        && a.getAttribute("title")) {
       a.disabled = true;
       if(a.getAttribute("title") == title) a.disabled = false;
     }
   }
}