A CSS question

I will apologise, in advance, if this post should be under another category.

I'm using Sigil (latest version) to edit a project that's a scan of an old title, to just make it look a bit smarter (mainly alignment issues).

Some of the Chapters have a main title, but then sections broken down with Roman Numerals as dividers. At present they look too close to the preceding, and following, text.

In the CSS stylesheet, I see this for the appropriate lines:-
display: block;
font-weight: bold;
text-align: center;
text-indent: 0;
margin: 1em 0 1em

My question is, why does the margin only have 3 entries?
(Shouldn't there be 4, for Top, Bottom, Left and Right?)

If not, which entry refers to which position?

Thanks