An MS themepack consists of sounds, walls and a modified color scheme for Aero.
Are you asking how to modify Aero into a wba? Well, that's what skinning is all about. However, a .theme file is an .ini text file that is divided into sections, which specify visual elements that appear on a Windows desktop. Section names are wrapped in brackets () in the .ini file.
Your WindowBlinds 7 should have come with already Modified Aero themes.
You can read more about themepacks in the MS library: http://msdn.microsoft.com/en-us/library/bb773190.aspx