How to remove the default background in Mediawiki?

Assuming that you are going to make changes to the monobook skin, go to skins/monobook/main.css, and comment out the background line in body like following.

body {
       font: x-small sans-serif;
       /*background: #f9f9f9 url(headbg.jpg) 0 0 no-repeat;*/
       color: black;
       margin: 0;
       padding: 0;
}