changing paragraph spacing

Hi. I’m trying to change paragraph spacing in my theme (Lean and Clean: Arizona) based on previous advice you’ve given to do this within the editor and style sheet. I’m a newbie, though, and my style sheet doesn’t seem to have the same coding as suggested in the other posts I found describing this.

Closest I could find is under the “MAIN CONTENT FORMATTING” section and looks like this:
#blog {
width: 480px;
float: left;
margin: 10px 0px 20px 10px ;
padding: 0;
font-size: 1.1em;
}

I tried changing the margin setting numbers and it didn’t seem to do anything. Am I in the wrong place? Thanks in advance for your help!