This is a Nintendo 3DS only theme, not the Switch!
Install Anemone3DS or Themely.
Put the two folders inside the zip into your "Themes" folder on the root of your homebrewed 3DS' SD card.
Next, boot the theme app and download one of the themes you want.
There! You have successfully downloaded this!
Knuckles Theme Preview: https://fileleaks.com/uploads/5fb515d5228d2/preview.png
Sonic Theme Preview: https://fileleaks.com/uploads/5fb518db8fde8/preview.png
MUSIC USED HERE BELONG TO THEIR RESPECTED OWNERS!