The directory /usr/share/phpbb3/styles/ is exclusively for packages
supplying styles (themes) for phpbb3. prosilver and subsilver2 are distributed
by the 'phpbb3' package itself.

The phpbb3 packaging will take a look in /etc/phpbb3/styles/ what templates
are available, so that you can add your own templates in /etc (as symlink to
somewhere else, or just the template itself). Template packages should put
their template in /usr/share/phpbb3/styles/, and make a symlink from
/etc/phpbb3/styles/.

An important consequence is that just adding a template to
/usr/share/phpbb3/styles/ alone _will not work_.