The Debian phpbb3 package uses /usr/share/phpbb3/www/styles as its default styles directory.
It is a symlink which points to /etc/phpbb3/styles, compliant with the FHS.
Installing custom styles should be done in /etc/phpbb3/styles, and already includes symlinks to the styles included in the phpbb3 package.
Regarding the styles directories:
The Debian phpbb3 package uses /usr/share/ phpbb3/ www/styles as its default styles directory.
It is a symlink which points to /etc/phpbb3/styles, compliant with the FHS.
Installing custom styles should be done in /etc/phpbb3/styles, and already includes symlinks to the styles included in the phpbb3 package.