Microsoft has a nice collection of controls bundled into a package called "IE Web Controls". Except for the fact that they are fully unsupported some controls are very handy.
I have used the TreeView in a custom WebPart and had some difficulty setting the styles. It seems you have to set the styles DefaultStyle, HoverStyle and SelectedStyle. The DefaultStyle is used when no action takes place, the HoverStyle is set when you hover with your mouse over an item and the SelectedStyle is the style of a selected item. Here are two examples of setting the style in a TreeView.
<mytree:treeview runat="server"
ID="tvwResources"
defaultStyle="font-family:Verdana,Arial,Sans-serif;color:#000000;font-size:11px;"
hoverStyle="font-family:Verdana,Arial,Sans-serif;color:#000000;font-size:11px;"
selectedStyle="font-family:Verdana,Arial,Sans-serif;color:#000000;font-size:11px;>
</mytree:treeview>
Or programmatically
mytree.DefaultStyle["font-family"] = "Verdana,Arial,Sans-serif";
mytree.DefaultStyle["color"] = "#000000";
mytree.DefaultStyle["font-size"] = "11px";
mytree.HoverStyle["font-family"] = "Verdana,Arial,Sans-serif";
mytree.HoverStyle["color"] = "#000000";
mytree.HoverStyle["font-size"] = "11px";
mytree.SelectedStyle["font-family"] = "Verdana,Arial,Sans-serif";
mytree.SelectedStyle["color"] = "#000000";
mytree.SelectedStyle["font-size"] = "11px";