Creating custom themes for the 3DS

Creating custom themes can be done in a number of ways using different tools such as Kame Editor. To use these utilities, you need to have CFW, a custom theme manager (e.g Anemone3DS) and a device that can manipulate files on a SD card.

Theme Editors

Kame-Editor - For Windows, Mac and Linux users (recommended)
Usagi3DS - For Windows users
YATA+ - For Windows users
YATA+ online - A browser version of YATA+

Should I not use the other editors?

You're free to choose which editor you'll work with, but Kame-Editor is the most user-friendly, which means it has an easy to understand interface that'll allow you to make themes with more ease compared to Usagi3DS or YATA+.

Another thing to keep in mind too is that Usagi3DS and YATA+ are incredibly outdated. Kame-Editor receives updates fairly regularly, and hence why you should use it instead of the other theme editors.

Instructions

What You Need to Know

Keep mind of the gap size. Each 3DS has a gap size between each screen, and that gap is approximately 56 pixels. If you want to use a full image for both screens, the gap size is very important.
Music for the theme must not surpass the file size of 3.3mb. Otherwise the theme will either not appear in your custom theme manager or it will simply not play music.
SFX (sound effects) have to be under the 182kb limit of file size. Be sure to use small SFX so you can include all the SFX you want to in your theme.

Using Kame Editor

For more information on how to use Kame Editor, refer to the official Documentation.

Using Usagi3DS

For more information on how to use Usagi3DS, refer to this guide.

Using YATA+

For more information on how to use YATA+, refer to this guide. This guide is valid for both app and browser version.