Additional CSS for individual posts and pages

Hi, thanks for developing a great theme! I’ve been trying to add custom CSS to style individual pages or posts, and so far haven’t been able to get this to work – whatever I do seems to affect everything or nothing.

For example, I would like to add a different image to the hero section on each page. I’ve managed to do this with CSS on each individual page but not in the main custom CSS, which isn’t a great way to do it as the WordPress editor tends to mangle CSS and it’s not good practice anyway. I’ve tried the obvious classes such as .page-id-xxxx for a page or .post or .single to style all blog posts, but these don’t appear to work. Can you help?

Many thanks ?? David.

The page I need help with: [log in to see the link]