Right, yeah, I was just trying to make clear that something like your original p { margin-bottom: 50px; } will work on a web page and maybe Reading view, but probably not do what you want because of the way Obsidian is structured.
The --margin-bottom in your second screenshot doesn’t exist, so that’s not going to do anything.
Did you try the paragraph and heading CSS I linked to? That should cover Reading View at least (for the spacing of paragraphs after each other). You might not see much in Live Preview depending on what else is in your note.
You could give this a try as well: