Thanks! To change the background, just find this in the code:

body {
background-color: #ECECEC;
color: {color:Text};
font-family: arial;
font-size: 10px;
margin: 0 auto 0 auto;
line-height: 1.53em;
max-width: 775px;
}

and add this above that final bracket:

background-image: url('YOURIMAGEURL');

Now, making the body and sidebar to stay white is going to be a bit tricky, since Nearly Witches wasn't designed that way. I'm guessing you want all of them to be white--what I mean is, the posts and the sidebar are inside one big chunk of white space. So from the top of my head, you need to find this in the code:

#everything {
margin: 50px auto 0 auto;
padding: 0 0 20px 0;
max-width: 775px;
min-height: 350px;
font-size: 10px;
}

and again, above the final bracket, add this:

background-color: #FFFFFF;

I hope that does it. I didn't Preview that or anything, so if it doesn't look like the way you imagined it, send me a new question and explain further what you want to happen, and I'll be glad to help. :)