Who designed the Holly wreath nixi?

None other than Kittykatblue!

They submitted this design back in 2016 :3

The Winter Fest 2020 list (which is confirmed by Krin) says the Holly Wreath Zolnixi is an Admin design.

(I was absolutely baffled by the answer that is was someone called 'Kittykatblue' because fest themes are taken from... well. The theme design thread of that fest for that year, or done by the admins. Why would Krin suddenly randomly go back to a thread from 4 years ago for one? So we went and hunted down the relevant post to see for ourself what their design was.
(Event Themes are occasionally drawn from a seasonal theme design thread also, such as at least one of the aeridini themes from easter last year being selected from the Spring theme design thread)
Kittykatblue did submit a design called "Holly Wreath" zolnixi back in 2016, but there's zero actual colour overlap between that design and the actual theme.)

(Kittykatblue's 2016 winter submission

vs 2020/2021 winterfest theme
)

I've seen quite a few of your answers on this forum and seriously the amount of research in each one is amazing