In loginstyle.php as well as in style.php there is no charset in the header()
for the stylesheets.
Unproper configured servers may add a wrong charset what would result in no
backend stylesheet.
(especially in Firefox)

Recently there was an issue where the server added charset=none to the header
what results in that Firefox did not load the stylesheet in backend.

somthing like 

header("Content-type: text/css; charset=" . get_encoding());

in admin/style.php and admin/loginstyle.php should do the trick.
(right after all required php scripts are included)

cheers