After reading Ruth's wonderful thread "Following the Christmas Lights Across Europe ", it occurred to me that this might be fun on a future Dec trip to Paris.
I'm sure you'd find more than one that does decorate; see for instance https://www.chambord.org/en/ and aslo https://www.chenonceau.com/en/agendas/christmas-at-chenonceau/. Gardens may of course be less impressive, with fountains off, and some trees even wrapped up for protection.
Chenonceau and Azay le Rideau both decorate for Christmas, as do Amboise and Cheverny. Of the 4 chateaux mentioned, Chenonceau is the most spectacular