Things You'll Need
Instructions
Open an Internet browser, and use a search engine to find R4 compatible Nintendo DS themes. Websites like NDSThemes offer hundreds of free themes.
Download your desired theme, and save it to your desktop. Insert your MicroSD or MicroSDHC card in your MicroSD card reader, and insert the card reader in an open USB port on your computer.
Click "Start," "Computer" and find your MicroSD card amongst the list of drives. Double-click it, and open the "_system_" folder. Open the theme's folder on your desktop, and drag the items into the "_system_" folder. If asked, replace the files of the same name.
Take your MicroSD card out of the reader, and place it in the R4 cartridge. Place the R4 cartridge in your Nintendo DS, and turn it on. The new theme is now installed and displayed.