Tumblr has flipped the switch and made HTTPS available on individual Tumblogs, not just the Dashboard. Further details are available at the link above. You should enable it!
Print Theme has been updated with a small tweak to the Twitter follow button to ensure HTTPS is maintained. The fix to the theme has been submitted and to be pushed out via Tumblr.
If you added third party scripts to Print Theme you will have to update the src URLs to HTTPS to avoid mixed content issues. Modern browsers will do their best to block some mixed content to maintain HTTPS, but its best if everything is secure.
If you remain HTTP or move back to it (you shouldn’t), this change wont cause any issues, the fix is backward compatible.
Enjoy browsing securely.