"CSS Buttons" has a particular meaning in Zen Cart as an option for replacing the conventional image-based buttons. However, it seems to me that you are simply looking to turn your EZ-Page links into buttons using CSS.
There are a couple of good tutorials on this. This one from Sophie G shows how to do this using background images. If you're just after a bevelled effect, then this tutorial may be more appropriate.
For some techniques you will need the ez-page links to be in a unordered list rather than just a list of a tags. To do this you would expand lines 20-24 of tpl_ezpages_bar_header.php to read
PHP Code:
<div id="navEZPagesTop">
<ul id="navEZPagesTopList">
<?php for ($i=1, $n=sizeof($var_linksList); $i<=$n; $i++) { ?>
<li><a href="<?php echo $var_linksList[$i]['link']; ?>"><?php echo $var_linksList[$i]['name']; ?></a></li>
<?php } // end FOR loop ?>
</ul>
</div>