When you have a phpBB2 board system installed, unpack the template package. There will be a folder named "Forum_XXXX" (where XXXX - is a four digit number). This is a phpBB2 theme. Place the files in their own directory ("Forum_XXXX" ) in the "templates" directory inside your phpBB2 directory. If your server is online somewhere, then you'll first need to unpack to a directory, then upload it to your server. Do not rename "Forum_XXXX" directory.
Go to your administration control panel.
"Styles Admin", click on the link
You will find a list of styles that are present in the "templates" directory but that haven't been installed to your forum. Behind every style, you see an "Install" link. Click on that link to install the style.
If you've done this for all the styles that you wanted to install, then have fun! They're now available to your users, or to you to set as the default style in "General Admin > Configuration".