Sounds like either sd corruption or the archive bit issue, check if this soultion works for you.Hi,i try this method
Download the NXthemes installer homebrew (the nro file) and copy it in the switch folder on your sd card.
Then make a folder called themes in the root of the sd card and copy your szs or nxtheme files in there.
Run from your favorite cfw the installer and use it to install your themes.
But my switch crash when i apply the theme,i'm on 6.2 sx os,try with atmosphère too,but not luck