Wrapping main content with a shadow

I am new to Wordpress and still finding my feet, so this may be a question that should be obvious to me.
I have a Wordpress blog created using a free theme. At present it is a bit on the boring side visually so I want to make the main content look a bit more attractive by having a shadow run down its length on either side. In websites I just create an image 30px wider than the content and repeat it on the y axis. How would I do this with Wordpress?

This could just be reduced to a simple CSS issue, depending on circumstances. Place the background image in your images file, then in your style sheet you could make the image repeat down the <body> element:

body {background: url(images/image.gif) repeat-y 50% 0;}

If it doesn’t work to have it on the bod (as that will repare all the way down the screen) then you’ll have to modify the template a bit first.