get firefox and the developer add-on or the firebug - great tools to debug and try changes to css and html.
all this has only very peripherally to do with wordpress - it is (basic) css tweaking.
in style.css:
obviously, the top of the page info and navigation is styled with:
#topnavbar {
width: 960px;
height: 34px;
color: #FFFFFF;
margin: 0px;
padding: 0px;
}
#topnavbar a, #topnavbar a:visited {
font-size: 11px;
font-weight: bold;
color: #FFFFFF;
text-decoration: none;
}
and all of the under header navigation is styled with the styles starting at:
#nav li a, #nav li a:link { ....