Can’t style comment popup, please help

I need help trying to style the popup that opens up using the comment popup function. I’ve tried everything to identify the elements in the css to style them, but nothing works. I can post the code if everyone would like, it’s the standard code in the comments_popup.php. For example, the title in the popup reads

<h2 class=”popup_h2″>Comments</h2>

I’ve tried calling out the h2 class without any luck. Do I have to style it inline instead of using the style.css file? I’m relatively new to designing wordpress themes, so if I’m doing something really newb, please forgive my ignorance. If anyone has experience styling the popup, I would appreciate any help you can provide. ??