Background can be set common for the whole site, different for each category or page type, or even unique for each page or post. It can be customized in several ways – have a look at several fitting methods below. You can also set it to scroll with the page, or stay fixed to screen. On top of it, there is stripes effects to apply.
There are also handy tools to automatically use images used in posts and pages as a background.
With all these options, you can achieve a range of effects. Have a look at a few examples: