In the stubfile, you can put this (it is allready in there !)

	# Now, select the default skin you want to display for this blog.
	# This setting refers to a subfolder name in the '/skins' folder 
	$default_skin = 'custom';
	# You can *force* a skin with this setting:
	# $skin = 'fplanque2002';

You can also go to the backoffice from your blog and put there the default skin.

Thanks. :D