Spydie, you're confusing things.
The path for pearl* styles .css is /templates/mg_themes/
The .css needs to go to /mg_themes (DON'T USE theme_info.cfg) as it will overwrite the main theme_info.cfg, which already contains the pearl* cfg)
The pearl image folders go to /mg_themes/images/
And if it is not done this way - the script will not be recognised or acted upon, for either black or blue pearl; because if it cannot find any one of the lines (Paths / Files) below, as far as IP is concerned the style doesn't exist.
$mg_themes[$tc]['template_name'] = 'mg_themes';
$mg_themes[$tc]['style_name'] = 'Black Pearl';
$mg_themes[$tc]['head_stylesheet'] = 'style_p_black.css';
$mg_themes[$tc]['body_background'] = 'p_black';
And the sooner MG gets away from this "Nested" template system, there will be a hell of a lot less confusion as to what goes to where.