Try this:
div.AspNet-TreeView {margin: 2px 0px 20px 0px;padding: 0;}
div.AspNet-TreeView ul { list-style: none; margin: 0px 0px 0px -20px; padding: 0;}
.AspNet-TreeView-Show { }
.AspNet-TreeView-Hide { display:none;}
div.AspNet-TreeView ul li
{padding: 3px 0 2px 0; }
div.AspNet-TreeView ul ul { margin: 2px 0 2px 10px;}
div.AspNet-TreeView ul li img
{border: none; vertical-align: middle; margin: 0 2px 0 0; }
div.AspNet-TreeView li a { margin-left: 0px; padding-left: 0px; text-decoration: none;}
div.AspNet-TreeView li.AspNet-TreeView-Root {}
div.AspNet-TreeView li.AspNet-TreeView-Parent {}
div.AspNet-TreeView li.AspNet-TreeView-Leaf { background: transparent url('go.gif') no-repeat scroll 6px 6px; margin-left: 20px; padding-left: 25px; }
div.AspNet-TreeView .AspNet-TreeView-ClickableNonlink { cursor: pointer;}
div.AspNet-TreeView .AspNet-TreeView-Collapse
{background: url('collapse.gif') 0 0 no-repeat; width: 5px; padding: 7px;
float:left; cursor: pointer; }
div.AspNet-TreeView .AspNet-TreeView-Expand
{background: url('expand.gif') 0 0 no-repeat; width: 5px; padding: 7px;
float:left; cursor: pointer; }
div.AspNet-TreeView a,
div.AspNet-TreeView a:visited,
div.AspNet-TreeView a:active,
div.AspNet-TreeView a:link {color: #4F82CB;}
div.AspNet-TreeView a:hover { color:#4EBF37;}
div.AspNet-TreeView .AspNet-TreeView-Selected a
{font-weight: bold !important;color: #4EBF37 !important;}
div.AspNet-TreeView .AspNet-TreeView-Selected ul a
{ font-weight: bold !important; color: #4F82CB !important;}
li.AspNet-TreeView-Leaf,
li.AspNet-TreeView-Root ,
li.AspNet-TreeView-Selected ,
li.AspNet-TreeView-Parent
{display: block;list-style:none;font-weight: bold;color: #333;text-decoration:none; padding: 2px 5px 2px 10px;
background: #f2f2f2;border-left: 5px solid #CCC;border-bottom: solid 1px white;border-top: solid 1px white;min-height: 18px; }
li.AspNet-TreeView-Selected ,
li.AspNet-TreeView-Leaf:hover,
li.AspNet-TreeView-Root:hover ,
li.AspNet-TreeView-Selected:hover ,
li.AspNet-TreeView-Parent:hover{border-left: 5px solid #4EBF37;}
li.AspNet-TreeView-Root { margin: 0px 0px 0px 20px; }
li.AspNet-TreeView-ParentSelected ,
li.AspNet-TreeView-Selected ul li.AspNet-TreeView-ParentSelected
{ margin: 0px 0px 0px 0px; }
li.AspNet-TreeView-Leaf img,
li.AspNet-TreeView-Root img {display:none;}
li.AspNet-TreeView-Leaf a,
li.AspNet-TreeView-Root a ,
li.AspNet-TreeView-Selected a ,
li.AspNet-TreeView-Parent a
{padding: 0px 0px 0px 10px;color: #333;}
You'll notice I removed the background from the div.AspNet-TreeView li a selector and adjusted the padding and margin on the div.AspNet-TreeView .AspNet-TreeView-Collapse and div.AspNet-TreeView .AspNet-TreeView-Expand selectors.
HTH,
Joe D.