Header not obeying stylesheet

I’ll admit that I’m pretty new at a lot of this, so if this is a completely stupid question, feel free to point and laugh. 😉

I’m building a theme to use for a new site for the company I work for. Most things are working just fine, but the nav bar completely ignores all of the styling in my stylesheet. I’ve gone in and added some styles inline, but that is such a pain and I can’t figure out how to add things like “hover” inline anyways.

This is how I have linked to the stylesheet:

<link rel="stylesheet" href="<?php bloginfo('stylesheet_url'); ?>" type="text/css" media="screen" />

The stylesheet has all of the correct information at the top (I think…), and as far as I can tell, it’s all written correctly.

What am I doing wrong? I’ve never built a site from scratch before, so this has been a steep learning curve.

The site is http://jesstest2.xtreemhost.com/ if you want to take a look. (And please ignore the horrific color scheme and positioning issues. I’m still ironing out the kinks and my monitor apparently needs a calibration…)