document.bgcolor property.

document.bgColor="#FFFF99"  // set to gray

code:

<form name="bgcolorForm">Try it now:

<select onChange="if(this.selectedIndex!=0)

document.bgColor=this.options[this.selectedIndex].value">

<option value="choose">set background color    

<option value="FFFFCC">light yellow

<option value="CCFFFF">light blue

<option value="CCFFCC">light green

<option value="CCCCCC">gray

<option value="FFFFFF">white

</select></form>