I agree with you: the Themes section in the SilverStripe addons leaves somewhat to be desired - since there aren't many ready-made themes and (almost) no screenshots or categories to help the search - especially if you compare this to, say, Wordpress...
This is probably because a lot if not most of SilverStripe developers start from or boilerplate template, or from static HTML/CSS and build the SilverStripe Controls into them. Hardly anyone uses a SilverStripe site out of the box: modules are added, custom code and custom fields and they all need to be incorporated in the theme, so often you're better off that way... It's not so hard to turn a HTML page into a SilverStripe template: https://www.silverstripe.org/learn/lessons/migrating-static-templates-into-your-theme?ref=hub