Copying a drupal default theme to /sites/all/themes/ and rework the .info, template.php and theme-settings.php as per tutorial and all works - until!
Until one inserts the line base theme = garland or base theme = new name into the .info file of the new renamed .info. Bingo drupal is dead, no access to the system because there's no theme.
It does not default to (Seven) the admin theme. Most peculiar that drupal can eliminate access because of a line in the .info file. If the offending line is commented out, deleted or changed to garland, drupal still dead.
It seems that drupal reads the multi site .info once and saves a change to somewhere in the database then does not re read the .info file To fix: Needs a fresh install of that particular multi site.
So if someone changes the .info file it kills drupal 7! This needs serious attention. It is inconceivable that something as innocuous as the .info file can be so destructive.
Cheers Roger