var currentTime = new Date().getHours();
if (currentTime >=6 && currentTime < 18) {
    if (document.body) {
        document.body.background = "http://link.pl/img.jpg"; //link do obrazka dzień
    }
}
else {
    if (document.body) {
        document.body.background = "http://link.pl/img.jpg"; //link do obrazka noc
    }
}