The invitation should make clear what the theme is. The design itself can be utilized as the first message bearer. Proper selection of wording will also play a role in it. The invitation could state the following:
It's A Baby Shower! Bottles and booties, diapers and pins this
is where the love begins!
(name of mom-to-be) would love to share with you a special occasion.
Please join her for a diaper-themed baby shower on (date) at (time
and location)
Please RSVP to (name of person and contact information)
To make (name of mom-to-be) happy, please bring in diapers ad baby
lotion gifts.