Hide background header image on specific page

Site URL: http://www.georjart.com/paintings-new

Hello,

The password for the page is: M6NYey7f

I've added a background image to the header of a specific page on the website above using the following code: 

#collection-6058d724f45e96664a810213 header#header {
    background-image: url(https://www.emergentcreative.co.uk/wp-content/uploads/2021/04/Gart-BG-Header-Example-1-2.jpg);
    background-size: cover;
    background-position: center center;
    background-repeat: no-repeat;
  position: absolute;
    top: 0;
    right: 0;
    margin-bottom: 9px;
    padding: 40px
}

However when I go to mobile view it looks like this:

I can see that the issue is with the "position" and "background-image" code and by going into Dev Tools in Chrome I can edit the following media query: 

@media only screen and (max-width: 750px)
#header {
    background-image: none !important;
    position: inherit !important;

}

I can get it too look normal: 

However I'm not certain how to add this code into the CSS editor in Squarespace, let alone on a specific page which is what I'd like to do since the changes aren't live yet. 

Does anyone have a solution? 

Cheers, 

M. 

Hi. The url doesn't exist. Can you check again?