Hi,
Re install the theme. Check the index.php file if you find the following code:
<?php
define(‘WP_USE_THEMES’,true);
/** this will load wordpress templates **/
require (‘./wp-blog-header.php’);
?>
If you don’t, then add them.
To check whether you have .css file, you can go to “Theme” folder, and click at the theme you just installed (it should be a folder), and check if you have style.css file in it.
If everything is ok, then activate the theme, and deactivate the previous one.