Help changing Header styles

We're trying to remove/reduce the large space placed after headings. We'd like the regular body text to start pretty much immediately following an <h4> (for instance). We like that there be space ABOVE the headings.

I thought that inserting

"h1, h2, h3, h4, h5, h6 {
margin-top: 10px;
margin-bottom: 0;
padding-top: 0;
padding-bottom: 0;
}"

into "CSS Inserts" would override the "margin-bottom: 10px;" I'd found in the "style.css" file, but the space following the <h4>'s is still there.

Do I have to actually edit the "style.css"? Should we EVER be making changes to "style.css"??

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