He installed it.
For custom, user-made themes not found in the official archives, many users visit community sites like Theme Plaza . 3ds theme archive
You will need to follow a guide like 3ds.hacks.guide to install and Luma3DS . This allows your system to run unsigned code. He installed it
To use custom themes from an archive, your 3DS must be running , such as Luma3DS. The standard tool for managing these files is the Anemone3DS Theme Manager . How to Install Custom 3DS Themes! This allows your system to run unsigned code
A 3DS theme archive is a curated collection of Nintendo 3DS themes bundled for preservation, sharing, or personal use. Archives can include:
Before March 27, 2023, you could boot up your 3DS, go to the Theme Shop, and spend a few dollars (or My Nintendo points) to download a theme directly. After that date, the ability to purchase new content was cut off.