proper way to add classes to the 'select all' button that appears below the header

I'm trying to find the best way to add some bootstrap classes to the 'select all' button that appears below the header row in tables. I was hoping to be able to do it with a simple hook_form_alter/hook_form_FORM_ID_alter, but couldn't find the form element in there (likely because it gets added with js).

Currently I'm overriding the entire theme_views_bulk_operations_select_all function which seems way overkill to add a couple of classes.

I should add that I tried function hook_views_bulk_operations_form_alter as well.

any suggestions would be much appreciated and return with a doc page for it ;-)