Share user database for 2 (or more) different forums.

[Alpay]

If you are not sharing ( dbprefix_themes )

After being a  NEW member ( register.php)  for a while error field;

'{db_prefix}themes', smf_themes
Sources/Subs-Members.php

// Theme variables too?
   if (!empty($theme_vars))
   {
      $inserts = array();
      foreach ($theme_vars as $var => $val)
         $inserts[] = array($memberID, $var, $val);
      $smcFunc['db_insert']('insert',
         '{db_prefix}themes',
         array('id_member' => 'int', 'variable' => 'string-255', 'value' => 'string-65534'),
         $inserts,
         array('id_member', 'variable')
      );
   }