Add theme hook suggestions to item_list

seems your theme callbacks are not defined in the module, maybe I forget something about theming

The portion of the theme hook after the first __ separator is a theme hook suggestion. The base theme hook, item_list is already provided by core. If suggestion(s) don't exist, then it falls back to the subsequent shorter theme hook suggestion and ultimately the base theme hook, e.g.:

item_list__facetapi__current_search_item_active   // <- looks for this fist
item_list__facetapi                               // <- then looks for this
item_list                                         // <- falls back to this

but looks like it will not work

It does work, theme hook suggestions have been around since 6.x. This isn't anything new.