How to change page background color

Hi,

I have question regarding my pleasant nail site. Attachment is the website page that divided into three section. the left and right side have black color and the page content in the middle have light yellow (see attachment). Can you please provide me some coding that I could change the color in the background of the left and right side 15% wide each side to have the same color green and the middle section to have color black?

Thank you
chad