The data of Simple CSS plugin is not reflected on the page preview until I save it.
Would you improve this behavior to see the result without saving it?
We don’t have a plan to improve this unfortunately.
I believe most of the users have switched to Additional CSS from WP to avoid an extra plugin.
If you still prefer simple CSS like me, then my method is to write everything in Additional CSS so I can see it change live then move it to Simple CSS once everything is finalized.
The Additional CSS isn’t useful as per page styling.
For the global styling, I would use style.css of the them instead.
The reason I use Simple CSS is for the per page styling.