Why did all my themes disappear when I upgraded to PHP 7?
I don't know specifically why your themes disappeared, but the requirements page for Drupal (https://www.drupal.org/docs/7/system-requirements/overview) states:


  Drupal 6: PHP 5.x only (5.2.5 or higher recommended). Warning: support
  for PHP 4.x has been dropped. Drupal core should work with PHP 5.3.x,
  but PHP 5.3.x and higher may produce errors or unexpected behavior
  especially for contributed modules and themes.


PHP 7 is definitely not supported. You will likely find many more errors than this over time.