Change site style based on users system time

1

Could anyone point me to a good tutorial or explanation on how one might change a site element based on the users computer time? What we are trying to do is change the color of the sky and shade of the buildings in a cityscape scene on a cartoonish style site. If it is 9:00 PM on the site visitor’s end, they will see a nighttime colored scene. If it’s 7:00 AM, it would be sunrise-ish. I figure something like this would be pretty simple, but for a newbie like myself I needed some help!