I made the catalog block themable.
I changed the following functions in "uc_catalog.module" to be able to handle theming.
and added the following theme functions (which can be overridden in your theme):
theme_uc_catalog_item($here, $active_link, $lis, $expand, $inpath, $link, $count_children )
I also added a "sequence" to class "uc_treeNode" (I need it in my own template, if you don't need it, it shouldn't be in your way).
It's a start, feedback very welcome (this is my first drupal contribution).