PHP limits "post_max_size" and "upload_max_filesize" are set to 32Mb for supscription. But it is not possible to upload Wordpress theme which size is larger than 10 Mb.
'upload_max_filesize' have "Definition of PHP_INI_* modes as PHP_INI_PERDIR" i.e.:
"Entry can be set in php.ini, .htaccess, httpd.conf or .user.ini (since PHP 5.3)"
However, Plesk implementation set up these values via windows registry. Follow the link to find additional information.
This issue was reported to Service Team with ID #PPPM-1895. The fix will be provided in one of the future product updates.