Hooking into an update of the db system table
You should be able to use hook_themes_disabled().

The following code in a module will put themes being disabled into the Drupal logs:

function MYMODULE_themes_disabled($theme_list) {
  foreach ($theme_list as $key =&gt; $themename) {   
    watchdog('MYMODULE', '%themename was disabled.', array('%themename' =&gt; $themename), WATCHDOG_NOTICE);
  }
}


EDIT:  Forgot to mention this, but hook_themes_enabled() is used in exactly the same way, so you could easily just modify the hook above to check when a theme is enabled.