Style Sheet Missing

Thanks for taking the time to assist me. I have searched other answers to this seeminly common question, but have not found the gold I am looking for.

http://www.prepyourkids.com is my website. I have downloaded a few themes and used filzila to load them into the themes folder under wp-admin. I have extracted the files that needed that doing, have deleted the lot and started again – but I have the same error message.

In my WP manage themes area the themes come up normally, indicating to me that they woudl be complete – however when I click on them to use them as a current them- the little message at the bottom tells me that there is a stylesheet missing and when I go to the blog – it is blank.
only the default and the classic works. I have checked with each one to see if the files ahve come across and the style.css is def there in each one.
does anyone have a clue to what perhaps I have missed or done wrong?

I have even tried a style that my other WP uses and it comes up with style sheet missing.
thanks again for your assistance – I am getting rather peeved at myself