You should check the DynaTree documentation on Theming.
You would write a custom.css and add reference to the associated classes
<link href="skin-custom/custom.css" rel="stylesheet" type="text/css" >
And then include the reference to this class using one of the option explained in the documentation:
<li data="addClass:'custom1'">Node with custom class 1
<li data="icon: 'customDoc1.gif'">Node with standard CSS, but custom icon
You can set up your own custom icons and the path is set as an attribute for the tree: imagePath: "skin-custom/"
Copied most of the lines from the documentation, if anything more specific that you need help with, please elaborate. Hope this gets you going. Thx