How to change a background in a WordPress theme? All WordPress themes

You may change the background of your WordPress theme in theme options. For example, in the Business WordPress theme, you can change the main background here in theme options with a field "Page Background". It's well described in the manual of the theme, follow your WordPress theme manual to get it - it will look similar like in the example I mentioned.

You can also change the background for each section as in the example below.