The theme you wish to use must be enabled. Instead of selecting the theme to use in Settings you must hard code it into your module. Change line 49 in manage.module 

$custom_theme = 'manage';
  init_theme();

replace the word 'manage' with the name of your chosen theme

$custom_theme = 'multiflex';
  init_theme();

FYI: module name really should be "admintheme" - I changed it in my install..