CSS inheritance

Theme: Twenty Twelve child

I am developing a site on my local WAMP server, and want to remove the top margin so the header touches the top of the page. Using firebug, I can click on the html, <div class="hfeed site" id="page">, and see that the following CSS controls the top margin: body .site {margin-top: 3.42857rem;}. However, when I try to change this in my child theme stylesheet, save, and refresh…the top margin remains. Any ideas?
Here are some methods I’ve tried:
body .site {margin-top: 0rem;}
body #page .site {margin-top: 0rem;}
body .hfeed site #page {margin-top: 0rem;}
html body.home div#page.hfeed <—(this is what firebug gives me when I copy CSS path)