change post background color

Background color is set in the CSS code — the specifics depend on your theme and site. With no additional information, not possible to offer any specific help. Try using
Firebug to identify the relevant CSS code.

Also note that changes should be made in a child theme or custom CSS — so that your changes are not lost when the theme is updated.

@bob, the easiest method is to add some css code to the body class.For example if you want to change the post background color, just add a css code to your style.css file which should be something like this, you can use WPyogi’s suggestion for the same

.single-post{
background:#999;
}