Squarespace 7.0. developer mode mobile section width

Hi, I have a client who built her website in developer mode (7.0). Her banner (background) images across the site are not optimised and not properly showing on mobile, a lot of the text is missing, so I was looking for an option to disable mobile views, either on the banners only or across the whole site. Since she's not working with a template to disable it on, I'd need to use some CSS. Can someone help me with this?