subsilvercp creates a new phpbb template directory parallel to
the default 'subSilver' hierarchy. It not only copies everything to
the new directory, but also renames a couple of files and replaces
instances of the string 'subSilver' in various files:
Be careful when choosing a new name, make sure it doesn't contain
special characters, and it's long enough to not collide with some
variable name in phpbb.
When you're done, there's a new template directory under templates. To
install it, go to phpbb's admin page, select ``Style Add'' and click
``Install''. Then, go to the ``Configuration'' panel, scroll down to
``Default Style'' and select the new style from the dropdown list.
Also, check ``Yes'' on ``Override user style'' to make sure users get
the new style.