Most of the showers I've been to were themed to fit the nursery, that way the MTB can keep all the decorations and use them to put in the nursery.
Agreed. Technically I'd say the hostess decides the theme, but the considerate thing to do would be to check with the MTB for the reason stated above. My shower (and more importantly, the shower invite) was done in a safari theme, so I got a TON of safari-themed gifts. But our already-completed nursery has got nothing to do with anything safari.
We can use - and appreciate - the gifts, but I can't say I'm really enamoured with any of them.