From phpBB Development Wiki
style.cfg is the phpBB Style Configuration File and is required within a style's directory tree:
Located in /styles/<NameofStyle>/
This file contains the configuration information for the style for use within phpBB3 depending on the phpBB-version
name = NameofStyle copyright = © 2009 Name of Creator version = X.X.X
Version numbers usually follow the typical Version Numbering guidelines.
In 3.1 style inheritance is used and marked with parent=MyParent
name = NameofStyle copyright = © 2014 Name of Creator style_version = 3.1.1 phpbb_version = 3.1.1 # template_bitfield = lNg= parent = prosilver