Hi,
Firebug shows you the complete HTML output (and its CSS), no matter what moves on hover or not. Please see attachment.
Assuming that you want to make the category name on the dark background better readable while leaving the category name during hovering as is, you'll need to first target the correct element, plus be more specific with your CSS regarding the wrapping container, like e.g.
.hikashop_img_pane_panel > .hikashop_category_name > a {
color: #fff;
}
The code you added to your custom.css for the classes "span.hikashop_img_pane_panel, span.hikashop_category_name" does not work, obviously. Best delete.