Ok. So there IS a way to link directly to a theme in the new Tumblr Theme Garden, it’s just not all that intuitive or helpful. If you’re trying to link to a specific theme directly, you can link to one of two URL’s: it’s “Preview” page — which is a working version of the theme with dummy content, or it’s “Install” page — which is where a logged in user can choose to instantly install it.

For example, I’m using the following two links to point to my Photo Board theme… 
http://safe.tumblr.com/theme/preview/113
http://www.tumblr.com/themes/install/113

For the record, I think the Theme Garden needs some work. Wouldn’t it be so much easier if each theme just had it’s own page? This page would obviously have a URL… something like, oh say, tumblr.com/themes/113 — or better yet — tumblr.com/themes/photo-board. This page could also have a screenshot of the theme, the install and preview links of course, and also space for a brief description or any unique features or instructions. As it is now, the Theme Garden feels very limited and a bit incomplete. Don’t get me wrong… it’s an absolutely brilliant piece of work as a whole. And the way they’ve made installing new themes is VERY seamless. I just think the “Garden” itself could use some refining in order to make it more appealing for theme developers as well as useful and easy for users.