Well try to merge nav and navback, aka rename the css ".navback" to ".nav" then compleatly remove this in the html code:
<div class="navback"> </div>
Then if it works as i think it till then it will draw the background behind the <div class=nav> which contains the menu. If it works but just looks a little weird / off you just need to adjust positions and width/height etc to get the div to adapt the proper size then to place the menu exactly where you want it
To make it more versitile in the future if you are running into problems you might wanna split this section into 2 divs, 1 for the header area and 1 for the tabs, like so:
<div class=nav>
<div class=tabs>
<ul>
<h1><div class='home'><li><a href="#">Home</a></li></div>
<div class ='store'><li><a href="#">Store</a></li></div>
<div class='contact'><li><a href="#">Contact</a></li></h1></div>
</ul>
</tabs>
</div>
So that you can more accuratly controle the size/position of the whole header area and the tab area seperatly