Not an approved method...
There should be a better way to do this since I thought you're supposed to be able to change the default theme on a global basis. If you're in jam and are willing, you could rename the default theme to something else like &standard& or whatever you want to call it, and then make a symbolic link from &default& to bluelagoon.
mv /usr/local/cpanel/base/frontend/default /usr/local/cpanel/base/frontend/standard
ln -s /usr/local/cpanel/base/frontend/bluelagoon /usr/local/cpanel/base/frontend/default
This will just make &default& the same as &bluelagoon&, thus giving the users what you want them to have.