The screenshots which appear on our website, were taken from various themes available at WordPress.org. We didn’t go by any specific list, we made selections at random. These themes are all available to the public at no charge. You will find several to choose from here: http://wordpress.org/extend/themes/
Some themes don’t have great CSS for forms. Currently I am looking at spending an hour or two in the theme CSS customizing the look and feel of the s2member forms. It would make me a happy man if I could somewhere download some CSS that would enhance the look of the forms without relying on the WP theme CSS.
So sorry. Actually though, this is intentional. s2Member Pro comes with basic structural CSS to get your Pro Forms to function properly. However, we leave colors and other visual effects to your theme. If your theme does not come with enough style specifications to cover forms, you would need to add styles of your own through style.css, or through a Custom CSS configuration section of your theme perhaps.
It’s also possible to use Hooks/Filters with s2Member, to either remove the default CSS, or to add custom CSS of your own. If you would like to learn more about that, I recommend this FAQ section.
Pre Sale FAQs » How can I prevent s2Member® Pro from loading it’s default CSS?
As always, we appreciate the feedback on this topic. I’ll see what we can do to improve on this.