[peugeot405]

you should add those changes in additional css, then it will change for the whole site not only for the forum page.

In additional css add:

.body_wrapper {
    background: url("http://i47.tinypic.com/34z0gef.png") no-repeat scroll right bottom #000000;
    border-radius: 20px 20px 20px 20px;
    padding: 10px 20px;
}