This error is occurring due to a known issue in the Stylecatcher plugin code. To resolve this issue, you will need to edit the /plugins/StyleCatcher/lib/StyleCatcher/CMS.pm file from within the Movable Type installation directory in a text editor, and locate the following code which should be at line 273 in the file:
CODE
print STDERR "styles=$styles\n";
Comment out the above line by adding a "#" at the beginning of the line:
CODE
# print STDERR "styles=$styles\n";
Then save the edited CMS.pm file, and upload it back to the /plugins/StyleCatcher/lib/StyleCatcher subdirectory within the Movable Type installation directory on the server, making sure to upload the file in ASCII mode, and making sure that the uploaded file overwrites the existing file on the server.
After uploading the edited StyleCatcher CMS.pm file to the server, log in to Movable Type, browse to the Design > Styles page, try applying a theme, and see whether you still encounter this issue.