@Got_Eevees Why woud you continue to save, you are still on 11.3 right? If so, you can easily install CFW thought the soundhax exploid. Just follow these instructions: https://3ds.guide/homebrew-launcher-(soundhax)
Anyway, about the themes. I have to agree with @Deoxyz in that it is all pretty confusing.
As you might have seen the Gold and Silver ones were officially revealed after September 13th's Nintendo Direct and you will be able to get them by buying the games on the eShop in JP and US. EU appears to be left out for some reason, although I cannot verify this myself? It also appears that people in the US are having problems in where they do not get a download code on their reciet. It is also unclear if you will receive a theme code if you buy a game from an other online store, for example Amazon, or if you will receive a theme code if you buy a boxed copy of the game (I do think we can confirm now that these boxed versions do not come over to the US, since they did mention these editions in the EU Direct but not in the US Direct). Anyway, another way to get these themes (if you have a Japanese 3DS) is though My Nintendo Japan, both themes cost 20 gold coins each.
Then we have the "Cast theme", which to my knowledge is not announced officially yet. I can't seem to find much information about on how to get this one outside of Amazon Japan, but thanks to @Deoxyz it looks like you also get this one if you buy though Amazon US. No idea how to get one in EU though, could be Amazon as well, but in my country Amazon is not nearly as big as in the rest of the world, so I hope there will be another way to get it.
And then we also have the Celebi code for Sun, Moon, Ultra Sun or Ultra Moon which also got announced at September 13th's Nintendo Direct. Apparently you will be able to get if you buy Gold or Silver on the eShop. The same uncertainty applies about receiving this code, as in it is not clear if you get it by buying the game though the eShop, buying a download code from another retailer or if you buy the boxed copy.
I have to say all of this is a huge mess. Hopefully they will just put all 3 themes on the 3DS Theme Shop so that everybody that wants them can just buy them and doesn't have to deal with this.