What type of clothing do japenes empresses wear?

It's unlikely that members of the Japanese imperial family wore commoner type kimonos in historical times. Formal imperial garb didn't change much from the 10th century on. The most formal for a lady would be the multi-layer dress called the junihitoe. They might be available as expensive wedding rental gowns these days.

This isn't easy to recreate, so you need to improvise something easier. If you think a crown and tiara might do it, what's on the heads of traditional hinaningyo dolls are pretty much what the emperor and empress would have worn.