Why did all my themes disappear when I upgraded to PHP 7? Today I had to do some support on a Drupal 6 website and when I tried to refresh a CSS file, all the themes disappeared. When going to the theme page, nothing in the list at all. Not even the core themes.

This is a Drupal 6.x website and I am running it on my current development system which runs PHP 7.x among other things.

Why would my theme disappear like that?

Note: I tried clearly all the cache tables and click on the "Clear all Caches" and the caching is turned off in the Performance screen (so that way I know it's not that creating problems.)
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.