Download themes from:
http://www.parallels.com/products/plesk/themes/
To install the theme customized by the server administrator, issue the following command:
/usr/local/psa/bin/branding_theme -i -vendor admin -source <path/filename.zip>
To install the theme customized by a reseller, issue the following command:
/usr/local/psa/bin/branding_theme -i -vendor-source <path/filename.zip>