Theme Preview Not Working

I cannot preview any of newly installed wp themes like church_10, massivepress and massivenews theme. However, the old themes can be previewed. When I forcefully activate, I cannot see my site. The installation is in the root directory.
I even tried this
The problem exists at line 852 in the wp-includes/themes.php file:

$_GET[‘template’] = preg_replace(‘|[^a-z0-9_.-]|i’, ”, $_GET[‘template’]);

It also does this at line 861 for the style sheet.

$_GET[‘stylesheet’] = preg_replace(‘|[^a-z0-9_.-]|i’, ”, $_GET[‘stylesheet’]);

The problem is it is replacing everything that isn’t a character, number, underscore, period or dash. Changing the expression in these two lines to:

|[^a-z0-9_.-/]|i

fixes the problem. “

from the following thread “http://mu.wordpress.org/forums/topic.php?id=10485”

I am not a wordpress expert and would appreciate if someone can help me.