Your Background is the area of your site that is behind your Template.
To change your background, go to the 'Appearance' section of your Control Panel and click on 'Background Color/Pattern' in the Theme: Basics' area.
The Backgrounds are divided into two sections -- Patterns and Solid Colors. Once you've selected a Pattern, click 'Choose this background' and your changes will be saved.
Or, if you would like to select a Solid Color, you can either input the exact hexadecimal code of the color you wish to use by typing it in in the square field below the color wheel in the form of the pound sign and then the code, or you can use the color wheel to select a color. Move the larger dot around the color wheel to select the color, and then move the smaller dot within the color wheel to adjust the color further.
Once you've chosen a color, click 'Choose this background' and your changes will be saved.