When the WordPress theme directory was redesigned earlier this year, a common request from readers was the ability to favorite themes as you can with plugins. This feature was also suggested by Mario Pesehv two years ago.
On July 22nd, Dion Hulse committed a change with little fanfare to the WordPress theme directory that allows registered users to mark themes as favorites.
Registered users who are logged into WordPress.org will see a heart icon above the download button on a theme’s detailed information page. When clicked, the heart icon turns red and the theme is added to your favorites list. To see your favorites list, visit the theme directory home page and click the Favorites tab.
To remove a theme from your favorites list, visit the theme’s detailed information page and click the heart icon.
Unfortunately, you can’t install themes marked as favorite from the backend of WordPress since the Favorites tab doesn’t exist on the Add Themes page. However, I’ve created a ticket on Trac to see if it can be added in time for WordPress 4.4. Keep in mind that like plugins, themes marked as favorites will show up on your WordPress.org user profile.