A theme pack is a way to organize various themes and display a theme based on certain conditions. Currently, theme packs only uses the current month as a criteria, but this will be enhanced in future versions (such as event-based themes -- Christmas, New Year's Day, birthdays, etc).

For an example ThemePack file, Download Calendar.wtp. You can open it in a text editor to see how it works. Copy the ThemePack file into the same directory as the themes:
c:\Program Files\Aeris Calendar\Themes\
For this particular ThemePack, you will need the twelve themes designated to be used with the Theme Pack.