Hi Goran,

Of course it should be possible to set a background on the li or a elements in the treeview. Its just a matter of viewing the source of the page to see what classes are available to create a selector and the attach the rule with a background image.

CSS boils down to selectors and rules in the format
[selector] {rules}

When I view the source of a page using a skin with a vertical treeview like freecsstemplate-level2 for example. I see markup like this in my local test site:

<div class="AspNet-TreeView" id="ctl01_PageMenu1_ctl00">
<ul id="ctl01_PageMenu1_ctl00_UL">
<li class="AspNet-TreeView-Root AspNet-TreeView-Leaf">
<a href="/test-article.aspx" title="Test Article">
Test Article</a>
</li>
<li class="AspNet-TreeView-Root AspNet-TreeView-Leaf">
<a href="/files.aspx" title="Files">
Files</a>
</li>
<li class="AspNet-TreeView-Root AspNet-TreeView-Leaf">
<a href="/feeds.aspx" title="Feeds">
Feeds</a>
</li>
<li class="AspNet-TreeView-Root AspNet-TreeView-Leaf">
<a href="/gallery.aspx" title="Gallery">
Gallery</a>
</li>

...

So I can see that every li has the class AspNet-TreeView-Leaf, so one candidate for a selector to put a background image on the li is:
li.AspNet-TreeView-Leaf { background:url('whatever_bg.png'); background-repeat: repeat-x;} 

Note that the image should ideally be in the root of the skin folder and it should be single quoted (this helps our css combiner resolve the correct full url to the image).

Other selectors that might also get the job done are:

.AspNet-TreeView li {}

or maybe you will find it better to put the background on the link

.AspNet-TreeView li a {}

or

li.AspNet-TreeView-Leaf a {}

Hope it helps,

Joe