Sorry, I’m not sure why this would be a theme issue? There are several posts in the forum about getting Gutenberg images to be responsive (which is where I found the CSS that was working prior to version 5.9).
I’ve discovered that I need to use:
@media (max-width: 800px) {
.wp-block-gallery figure.wp-block-image
{
margin: 0 0 1em 0;
width: 100% !important;
}
}
…in order to affect the gallery blocks with the changes in version 5.9. This is working, however, the top images are indented.
My theme support couldn’t help because this is a Gutenberg thing.