Take a look at this topic, have several tutorials that explain how to create all items for customized themes.Ryumaru linked me to this thread yesterday, so I'm going to dump my themes here!
I'm VERY new to theme making and I feel like I should note that graphics are my main skill, when it comes to audio I am a complete rookie and haven't bothered to try out custom looping yet, so some themes may just have an end to start loop, although most should have custom loops since I grabbed a lot from smashcustommusic.
All of them use official art, in-game sprites, promotional illustrations, etc, and official music/sounds, in case that's your kind of thing. (or tiled backgrounds that I had to make for some of their lower screens.)
Digimon Adventure Theme
Basic theme, custom BGM (has a startup delay which I will fix at some point), no SFX.
Download link
Happy Happy Moumantai
Basic theme, custom BGM, no SFX.
Download link
HUNTER X HUNTER - Ending 6
Basic theme, custom BGM, no SFX.
Download link
Midnight Lycanroc
Basic theme, custom BGM, no SFX.
Download link
Zekrom
Basic theme, custom BGM, no SFX.
Download link
Reshiram (and N)
Basic theme, custom BGM, no SFX.
Download link
MOTHER 3 - Sunflowers Theme (v2)
Basic theme (but my personal favourite), custom BGM, custom SFX (MOTHER 3 menu sounds), and (lazily made) custom folders and borders.
Download link
MOTHER 3 - Final Needle (v2)
Basic theme, custom BGM, custom SFX (MOTHER 3 menu sounds).
Download link
MOTHER 3 - Tazmily Chapter 1-3 (v2)
Basic theme, custom BGM (minus the love theme segment, has a clean cut), custom SFX (MOTHER 3 menu sounds).
Download link
MOTHER 3 - Tazmily Chapter 4 (v2)
Basic theme, custom BGM, custom SFX (MOTHER 3 menu sounds.)
Download link
Team Kirby Clash Deluxe
Basic theme, custom BGM, no custom SFX.
Download link
Kirby - Dreamstalk (v2)
Basic theme, custom BGM, custom SFX (Kirby's "Hai!" when 3ds lid is opened.)
Download link
Kirby - Galaxy Parasol
Basic theme, custom BGM, custom SFX (Kirby's "Hai!" when 3ds lid is opened.)
Download link
https://gbatemp.net/threads/tutoria...e-quality-of-official-nintendo-themes.460823/