Hi,
Did you re-download the latest package of HikaShop 2.2.1 ?
Sometimes, when we discover some bugs which could be problematic for users, we made a new package of HikaShop with these new fixes without change the version number. It allows new users who will download the package to have directly the fix without having to wait to the complete new version.
So depending when you have download HikaShop 2.2.1, it could be interesting to re-download the package.
For your CSS, the "input" is the checkbox itself.
There is a "div" with the class "hikashop_filter_checkbox" which contains the checkbox and a label for the name of the filter option.
After that, each group have a little title, it's a "span" with the class "hikashop_filter_title".
Hope it will help you.
Regards,