It's a problem with the server terminating downloads before it should.
I think I figured out the problem.
When I visit the styles page (http://www.phpbb.com/phpBB/catdb.php?cat=12
), and use the "Download File" link from there - the file wont open.
But when I visit this link (http://www.phpbb.com/phpBB/viewtopic.php?t=406071
, and use the Download File link from there, I was able to unzip and extract the files, no problem.
In any case, thank you for the quick response. Wonderful support!