You need to find the download link and download it to your computer..
Then unzipp (de-compress) the file/style using winzip - you can download that from www.download.com
Then use your FTP and upload the decompressed style to new folder which you must make in your phpbb direcroty onj your database.keeping the structure.
Then you can go to your admin CP and select the new style and make it a default stye if you wish.
PS I'm gonna be here a while if you need any further help as i've been waiting 12 hours for some help with my cashmod mod.