Adding padding-top to a custom footer

I have installed a custom footer into my webpage. However, I would like to have more “white” space between the end of my last article and the start of the footer. Looking at the page in firefox using firebug I can easily find that the html is

<div class="custom footer"

Furthermore, using firebug I can easily add a padding-top, which drives the footer down on the page. Messing around with the layout changes the html to

<div class="custom footer" style="padding=top: 100px;">

also the following is added to the “style” tab, two tabs to the left of “layout” in firebug:

element.style {
padding-top: 100px;
}

However, I can’t easily figure out how to make these same changes to my style.css. Can I even effect these changes in my style.css or do I need to copy another file over?

Thanks for any advice that you can throw my way.