Installing a custom theme has never been easier!
Since we have free access to all files of the emulated NAND, we can directly replace the "theme file".
* csm file of your theme (mym needs to be converted to csm too, go here if you don't know how to do that)
1. On your emulated NAND, go to Title -> 00000001 -> 00000002 -> content.
2. In this folder, you should see some .app files. Find the app file that belongs to your system menu:
Once you found the correct app, all you have to do is to remove it. After that, copy your csm file into the content folder and rename it to 000000XX.app (the file you just removed, replace the XX with the correct number).