OK, I've isolated the problem, which is in the code of RokGZipper version 1.9. The header code is being written incorrectly for utf-8. The proper way is documented here:
www.w3.org/International/questions/qa-css-charset.en.php
Content-Type: text/css; charset=UTF-8
On lines number 330 and 342 the header code is incorrect:header("Content-type: text/css; charset: UTF-8");
which must be changed to that above. Can a moderator please bring this to the developers attention.