Mario Galaxy Wii Menu Custom Theme (DOWNLOAD) 

.mym file can be found here: drive.google.c...
DISCLAIMER: ONLY COMPATIBLE ON ORIGINAL WII HARDWARE. Use on a vwii leads to a semibrick. I haven’t tested how it works on different regions, use with caution.
To pack the .csm file:
1.) use the ThemeMii application to insert the mym
2.) get a copy of 00000097.app (US) using NUS-Downloader (download sysmenu version, keep decrypted .app files)
3.) click “create csm” in thememii, then click the directory to 00000097.app, and then the directory in which you want the .csm file to be sent.
This is a pretty makeshift tutorial and I don’t expect anyone to understand unless they’ve already added themes to their wii. For more help, it may be best to visit a video tutorial.