imagine that we have a list if items in $items array that it's theme function is list_item in specific page.
code is
theme('item_list', array('items' => $items));
but in my module, I want to alter the parameter 'item_list' and I want to use my custom render function 'custom_item_list'
I know that I must register and define custom_item_list function but I don't know in what hook I am able to alter the theme_function parameter of specific element.