Download seethrough theme
To use
Go to Dashboard > Customize > Theme, click on “Use custom HTML” (if you hadn’t already), then copy and paste the theme code into the text field.
To style
Go to Dashboard > Customize > Advanced and paste something like the following in the “Add custom CSS” field:
#page {
background: url(http://toys.tumblrist.com/seethrough/img/image1.png) no-repeat top left;
}
Replace the address of that image to whatever you like — Flickr, Photobucket, whatever. More css examples are available here.