One of the most helpful tools that I could find online was the ability to customize a specific block using CSS on Squarespace, thanks to the extension called: Squarespace Collection/Block Identifier.
If you want to take the best advantage of the CSS customize option on Squarespace, this is your MUST HAVE tool to add when starting to work on Squarespace. I wish someone would have told me that before. But oh well.
It is quite easy to get and much easier to work with. Once downloaded, you can just edit the particular section you have been wanting to work - all without having to change the whole website.
I hope this helps you on your way to perfecting your websites.