How to change the background image through the tag “selected”

Question

I\'m trying to change the background image of the body (id = \"shelf\") using a drop-down menu:

Answer 1

It's working.

function changeTheme() {
    var e = document.getElementById("themes");
    var theme = e.options[e.selectedIndex].value;
    document.getElementById("shelf").style.backgroundImage = "url(" + theme + ")";

}