Modify the JavaScript property document.body.style.background.

For example:

function changeBackground(color) {
   document.body.style.background = color;
}

window.addEventListener("load",function() { changeBackground('red') });

Note: this does depend a bit on how your page is put together, for example if you're using a DIV container with a different background colour you will need to modify the background colour of that instead of the document body.

If you want change background color on button click, you should use JavaScript function and change a style in the HTML page.

function chBackcolor(color) {
   document.body.style.background = color;
}

It is a function in JavaScript for change color, and you will be call this function in your event, for example :

<input type="button" onclick="chBackcolor('red');">

I recommend to use jQuery for this.

If you want it only for some seconds, you can use setTimeout function:

window.setTimeout("chBackColor()",10000);