go to softmii.com download the theme corresponding to your wii's type
all the 3.x are around 4x
while 4.x is 7x.
also look for the themes corresponding to your wiis region.
if you install a wrong theme you wii can be bricked so for backup I recommend preloader.
x = is a number depending on your region
use the app softmii tweak toy to install the files, do not edit their names it might be dangerous even though I haven't tried it.