I know until either the end of the month or some time in January, you get a Celebi code & 3DS Theme when you get Gold/Silver, but do we know if you get a Theme with Crystal? I mean being able to get a legit Shiny Celebi sounds cool, but that little bit extra in the Themes makes Gold & Silver better values overall to me.
There is no 3DS theme for Crystal, unfortunately.
Nintendo 3DS Themes (Serebii)
No, there has been no announcement on whether Pokemon Crystal comes with a 3DS theme.