WordPress keeps adding new features all the time. Unfortunately some of these new features may break the house of cards you built to make your website. This is true with the new “Custom Background” feature. While this feature was added in version 3.0, some users who upgrade to 3.4.1 may find that the custom background feature no longer works.
Just like many other WordPress problems, there is an easy solution, once you track down what the issue is. The problem is most likely that your theme needs to be updated. If you purchased a theme, then you may need to contact the author to update the code.
You can easily fix this yourself in a few minutes. You just need to update your theme’s <BODY> tag in the header.php file. Go to Appearance > Editor and then click on Header.php.
Scroll down until you see the <body> tag and then update that line like so:
<body <?php body_class(''); ?>>
Click on Save. If you don’t have write access to your server then you may have to manually edit the file from a shell prompt or even edit the code on your computer and upload it to your themes directory.
After you update the <body> tag then you should notice that custom backgrounds magically work again.