altering fixer theme to allow side ads using another plugin


i am trying to alter my site, on the sides to show an a background ad image, either side of my content sections i am making , using my plugin; WP Background Takeover. (it uses a specifed DOM element to insert content)

in order to do this i need to create an class/element that spans the background of the site, and is full width. But i also need to make column space on the left and right of the content in the centre.

How can i please redo the widths of all sections.
And how can I create an element/class to select for the plugin, to show an image in the background, that sites behind all website content

any chance anyone can help out with this please, as i wouldn’t be sure of exactly what im changing with wordpress.

many thanks!