The background is called the CSS on my adopts. This helped me change mine http://myadopts.com/forum/showthread.php?tid=119 Just replace the old code for your new one and if you mess up the code for the default CSS is on the thread. Hope this helps!
Or html. Try getting a few tips off from w3schools. They got all you got to know from html, css, php and whatever. Its a really good page, as you can try your code there and then see if the code works and how it is supposed to look like ^^
I'm unfamiliar with the setup of MyAdopt's preset CSS layout, but basically, look for this code within the body tag (most likely in the beginning of the code):