Hi there, have you got a url we can take a look at just to see what you have setup in the layout/sections etc.
All the sections will be full width if you've selected that in the layout editor. By full width it means the container holding the section content goes to the edge of the page, so if you applied a background color using css to that section it will go right up to the edge of the page. The content can be stretched right up to 95% as there will be padding/margins on the content, you can remove that with css and set the site to 100% width.
If you zap us a link to the site we'll take a look for you, let us know exactly which bits inside those sections you're looking to go full width or edit to make sure we're looking at the correct bits.