Remotely get WordPress theme version

Please check out this answer to a similar question about regex for getting info from the theme’s style.css.

When it comes to the second part of your question – if you are concerned about page load times, you could always use AJAX to get the information about each theme separately. You would display the page with all themes listed but without their version numbers at first. Once the page is loaded, you could send an AJAX request to wp-admin for each theme, process the information on the backend (get the theme’s CSS, run regex to find version number) and formulate a response with the theme’s version number. Display the version number on your page and you are done! Repeat for every theme on your list.