How do I access Wix Studio?

Question:
I have Wix studio access (apparently), I am able to get to the Studio dashboard, but how do I actually access the editor in Studio? I click ‘edit site’ and it takes me to the regular editor.

Product:
Wix Studio

What are you trying to achieve:
All I want to do is add css classes to elements so I can slightly alter the design, I thought adding Wix Studio would allow me to do that. If there is an easier way I am open to other suggestions.

What have you already tried:
I’ve searched every combination of terms I can think of, it seems like every article and discussion links out to other articles and discussions in a loop. All the videos I’ve found and Wix documentation assumes you are already in the Studio editor. But I just want to find out how to get into it.

Additional information:
thanks in advance for any help.

you have to create Wix Studio site from scratch. You cant edit previously designed sites from older editors in Studio

Ok thanks for the info. So is the only way to edit CSS on an old site to use the custom.css file, and not be able to view the changes until the site is published on a live URL?

What I need to do is adjust some spacing in a Gallery element, move inner elements like the text and thumbnails a bit.