That's pretty much it, create a folder called themes in the root of the sd and copy your themes in there, to install nxthemes you'll have to follow this quick guide first.
- Install the new nxtheme format
- Install the old szs themes
- Automatically detect proper name and firmware version for szs files
- Directly dump home menu nca, no more hacdiskmount !
- Suports all major cfws (atmosphere, reinx and sx os)
To know how to create your own custom themes and more info check out Switch theme injector's thread and the custom themes sharing thread.
Make sure to have layeredFS set up on your console or else themes won't show up.
Sx os doesn't require you to have the "sxos" folder on the root of the sd, but if you don't have it the installer won't detect sx os. So if you use sx os and don't have a folder called sxos on the sd, manually create it.
SZS files are illegal to share, don't post them here on gbatemp. You can only post nxtheme files.