Full-Width Post - help with CSS

Hi, I'm trying to create a post on my website. I used Divi library to create a layout, which I then translated onto a post. However I noticed that the whole layout is containerised in the middle and it does not expand to full width.
I am aware this is an issue other people had, see this: https://divi.help/threads/full-width-blog-post.5181/

The link above applies to the full post, whereas I only wish to have my header and a slideshow at the bottom in full width. I am happy with the body text. Is there another CSS code that I can selectively apply to those elements and expand them to full width? Where would I input this code, if so? For e.g the header (essentially just a banner image) has three CSS boxes in the advance menu settings.