How can I edit blog pages' custom CSS' globally?

Site URL: https://pelican-chiton-xwy4.squarespace.com/blog/esferica-vision-cities-sustainability

Hello,

I've created 3 blog posts at the moment and I'd like to do a few adjustments that'd affect them globally instead of having to keep adding specific "#block numbers" to each section's page individually.

For example, I'd like to add a "margin-bottom" to the first image (attached) so it doesn't overlap with the text below. I can of course do it targeting  #block-2ebecb79d07fed4c41d9 but this wouldn't apply to the other 2 pages since the #blocknumbers keep on changing for each page.

I would like my client being able to add a new Blog post with having to worry about the layout getting messed up across devices.

Is there any way that I can make adjustments globally to all pages??

Thanks!
Xavier