How to css Views Tree module theme
Finally, I discovered that the module uses the core listing resources, so the theming can be achieved via template.php and function NAMEOFTEMPLATE_item_list($variables). The only caveat is that, as the call to this function is recursive, you will need some tricks to know in which level you are. Those tricks could depend on the listing itself; another solution would be to modify the module itself to send the level.

Honestly, I think this in an example of a module whose existence is questionable. A direct treatment of the View_results is much faster and easier to theme.