Choose for the photo, but plan for the page
The best reveal themes are the ones that look good in a single shot: the moment the box opens, the colour the confetti throws, the faces behind it. That instinct is right, but a theme only earns its keep if it also reads clearly on the invitation and if you know how many people will be there to see it.
A note worth stating up front: a gender reveal has no dedicated product type of its own. You stage it on the baby-shower setup, the closest supported type, and pick from its designs to match your theme. The theme decides the styling; the baby-shower page carries the details and gathers the headcount before you spend on props.