For those who are having trouble, do not download by using the official launcher as that will unzip the file. Download it from a browser so you can have the zip file
Viewing post in SwatchBlingsBar Theme set comments
In short when you download from the official launcher, it’ll download as a regular file which that cannot be used when using it as a theme for your 3DS
If you download it from the browser, it’ll automatically zip the file which you can send to your SD card to your themes folder
So nothing really too complicated