Remove padding between menu and plugin

I want to remove a small gap between my main menu and a slideshow running directly underneath (I used the Post Highlights plugin). I’ve established the the space is actually the container of the slideshow, which adds 15px padding. I know i have to remove this padding… my question is where do I place it into my code… I’ve tried placing in #content, #header, etc. to no avail. I need to know what code I should add and where. I already know my issue is to remove the padding… i just need to know where to place the code…

The line containing the space is <div id="posthighlights_container">

The matched CSS provided by Chrome is as follows:

#posthighlights_container,
#posthighlights_container .ph-canvas {
width: 1000px;
height: 300px;
}
style.css:19
#posthighlights_container {
padding: 15px 0 0 0px;
text-align: center;
position: relative;
}
style.css:1
#posthighlights_container {
font-family: Arial, Helvetica, Verdana, Sans-serif;
}