Where are the CSS styles for the horizontals navigation at the top (and bottom) controlled? I want to change the color of the text. 

The top nav is a series of links in a div called nav-main.

So look in the style-sheet for this bit:

#nav-main a:link,
#nav-main a:visited {
	color:#FFFFFF;
	text-decoration:none;
}

#nav-main a:hover {
	color:#D4E7F8;
	text-decoration:underline;
}

#nav-main a:active {
	color:#FFFFFF;
}

As you can see, it is possible to have diferent colours for active, hover and visited links within that div.

They have done it this way so as not to override settings on other links outside of the nav-main div.