If you mean a NextSTART theme, you must first - if you haven't already - download NextSTART from http://www.winstep.net . Unzip the file anywhere and run setup.exe. Once the application is installed, you can fire up NextSTART preferences, navigate to the themes tab and export the theme zip file from there - NextSTART will take care of the rest.
Jorge Coelho
NextSTART 2.81 - A user interface you can actually use!
http://www.winstep.net - Winstep Software Technologies
IRC Chat: #winstep on AustNet