That CSS is for post loops.
Are you trying to make the images with extra-margin-bottom go full width on mobile?
If so, try this CSS:
@media(max-width:768px){
.wp-block-image.extra-margin-bottom figure{
float: none;
width: 100%;
}
.wp-block-image.extra-margin-bottom figure img{
width: 100%;
height: auto;
}
}
Note: This won’t go full-width of the viewport. It will only go full-width of its container like this: https://share.getcloudapp.com/ApuYN4xo