In your case, it looks like a gallery block, which is the hardest one to change.
You can use your browser Developer Tool to see what CSS rules affect the gallery, and write some rules to override those. Good luck…
How do I use a normal gallery and not the Gallery block?
]]>When you change the Additional CSS, it is put directly into the page, so there are no version numbers that will tell the browser that the page changed. You can hit Reload in your browser, but some browsers have to be told to clear their cache to get the new version. This will apply not only to you, but to every repeat visitor.
If you want to switch from Gallery block to normal gallery, you can use a Shortcode block with a gallery shortcode, or a Classic block with a normal gallery in it, or build your own in a HTML block.
Or you could use the Classic Editor (either per page or for all pages).