To change the theme, wallpaper, etc for root I believe you have to actually log in as root at the login screen.
I mean in ALL of the past 12 to 15 distros I have worked with, that is how I have always had to do it.
In fact, it is the only way. Since linux stores those kind of settings on a per user basis.
So, log in as root, and then change the themes, wallpapers, etc. and then log out.
Now, I do not condone logging in as root usually, but I also feel that in those very rare times when you must do so, why shouldn't the screen look like YOU want it to. I always make sure that my user account "llok" is specific to me, and the root accounts "look" is VERY distinct, just so "on sight" if you are in root or user.
Just please remember to log out of root after the changes. So far, I have yet to "customize" my root account in Mageia 4, but I shall soon be doing so. I like making the system truly mine.
Have a good one.