There is a readme, when you extract the mediocre zip file there is a mediocre folder, inside this is a folder called "readme-logopsd".
However i don't think the readme tells you how to install.
This article in the knowledge base
should tell you what you need to know.
Basically you extract the mediocre.zip file to your hard drive.
Which will leave you with a folder called mediocre containing files and sub folders.
You then upload (With your FTP client.)this mediocre folder and all of it's contents to your phpbb installs "templates" directory.
All the files, sub folders and files in the sub folders need to be uploaded. in the same structure.
Once that is done you go to the "styles admin" section of the admin panel and then "add". You should then see a list of all the styles you can "install".