Go to Dashboard >> Appearance >> Customize >> Theme Options >> Footer >>Style.
Choose Footer Layout from available options.
Choose Background Type from drop-down options. Background Type consists three options: None, Color and Image.
If you choose Color Background type then set the color for the Background and text.
If you choose Image Background type then Select the Background Image, Set the Overlay Color and Choose the Background Position, Background Attachment, Background Repeat, and Background Size from drop-down option.