Allow paragraph breaks and customizing the spacing

Use case or problem

Allow users to define the custom paragraph spacing, and make it visible in Live Preview. Currently, I have to press Enter (or Return) twice to get a line between two paragraphs. I wish to be able to create paragraphs with only one keystroke of the Enter/Return key. I’ve tried using CSS for this, but no solution seems to affect Live Preview mode.

Proposed solution

Allow users to define their custom spacing between lines, much like font size and line width are adjustable under Appearance.

Current workaround (optional)

I’ve tried any suggested CSS snippets. Nothing works on the default theme in Live Preview mode.

Which one do you think is the closest to your request?

What you’re referring to is the absence of Enter=New Paragraph (Enter=New Paragraph as default).
What you are describing as paragraphs are markdown lines. You can use CSS to change the space between lines, but they will still be lines and not paragraphs - which matters if you ever need to export.