It’s just as I thought in my first post, you’ve got nested lists so your CSS is not matching the output.
Your Wayfinder call should not be enclosed in <ul> tags so remove them from your template. You also don’t need the &rowClass parameter because your CSS isn’t referencing a class on the <li> tags. So change that to an &outerClass parameter. Your Wayfinder call should look like:
[[Wayfinder? &startId=`1` &outerTpl=`leftNav` &hideSubMenus=`true` &level=`1` &outerClass=`leftNavItem`]]
Then create (or modify) your &outerTpl chunk (leftNav) so it looks like:
<ul[+wf.classes+]>
[+wf.wrapper+]
</ul>
Wayfinder will replace [+wf.classes+] with class="leftNavItem" and your CSS should work.