customize popup

It depends on what you want to change.
If you need to change the form - use hook_form_alter.
If you need to change the style of popup, try to override file comment_abuse.js with hook js_alter

Seems your problem was solved.

How do you use css to change it? Im a beginner so don't know hook format

You can use hook_css_alter http://api.drupal.org/api/drupal/modules!system!system.api.php/function/...