There is CSS in the plugin directly.... I edited the default CSS by going through the plugin editor built into WordPress.
But I also found this tip here: http://beeex.net/web-development/wordpress/plugins/10-excellent-promising-wordpress-plugins-some-brand-new
What a few of you might miss is an option to style the content of the tab itself (not just the title). For instance, we added a light grey border to the content of this tab unit (it’s the border running around this very text). How?
The class for the tabs is postTabs_divs, and you can use it to style the tabs with CSS. Simply add this line to your theme’s style.css, and you get what we got right here:
.postTabs_divs { border:1px solid #dcdcdc; margin-top:-12px; }
The border explains itself, and the negative top margin was added so that the content div would match the position of the tab bar.