cover image full width

Hi

I am using directory starter theme and have put a cover image in but would like it full width so found a guide on how to do it https://johnregan3.wordpress.com/2018/12/19/how-to-add-full-width-images-in-gutenberg/ but it’s still not being full width image, it’s only the size of the div container by looks ok it.

It don’t seem to be applying the css I set the themes stylesheet, can anyone help please as bit stuck