"Purple and White" aren't a theme ... They are colors.
A "theme" is what ties the activities, colors, and decorations together.
What are some awesome ideas for a purple and white themed birthday party?
Have a "Party Like It's 1999". A Purple Rain Party. Purple Rain and 1999 are by Prince. We actually had this party in 2000. It was New Years. Everyone dressed in purples, and we danced to Prince. Now you could have everyone dress in late 90's dress, and decorate in purple. Have fun!