3DS:Custom themes/creation

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 online - An online theme editor

Should I use the other editors?

You're free to choose which editor you'll work with, but Kame-Editor is the best to date, having more features and support compared to Usagi3DS or YATA online. However, Usagi is still widely used among theme creators and is often recommended for beginners for its ease of use and simple UI.

Another thing to keep in mind is that Usagi3DS and YATA online are very outdated. Kame-Editor continues to receive fairly regular updates.

Instructions

What You Need to Know

Take note of the gap size. Each 3DS has a gap size between each screen, and that gap size is a different size for every 3DS model. Commonly used gap sizes are 50, 56, and 68 pixels. If you want to use a full image for both screens, the gap size is very important.
Background music for the theme, the bgm.bcstm, must not surpass 3371008 bytes in size, or the theme may not appear in your custom theme manager or may not play music. Always check the size in bytes, not in MB, because it's possible to have a bgm.bcstm be under the size of 3.3MB like several theme guides recommend, but be more than the limit of 3371008 bytes.
SFX (sound effects) for the theme must not surpass 182KB in size. Be sure to use SFX that are small so that you can include all the SFX you want to in your theme.
Do not make the background music mono. Even though this saves space, it can break the background music for the 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 online

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