To get the Famicom Mini and/or NES Classic themes, you will need to extract
them from your Famicom Mini or NES Classic Edition through hacking.

For the Famicom Mini, the theme graphics can be found in:
/usr/share/clover-ui/resources/sprites/hvc.png

For the NES Classic Edition, the theme graphics can be found in:
/usr/share/clover-ui/resources/sprites/nes.png

Just take the image, put it into the theme folder, and rename it "graphics.png".
For the Famicom Mini, it should go into
"Nintendo Classic Mini - Family Computer",
and for the NES Classic Edition, it should go into
"Nintendo Entertainment System - NES Classic Edition".

For the menu music, which is shared across both themes, look in
/usr/share/clover-ui/resources/sounds/hvc/
…take the files bgm_boot.wav and bgm_home.wav, and convert them into a single
MSU-1 track. Name it "bgm.pcm" and put it in your theme folder.