To help fuel the marketing hype of the upcoming Metroid: Samus Returns 3DS game, Nintendo created a unique Metroid theme for MyNintendo users. For the cost of 20 Gold Coins, MyNintendo members can download this theme for use on any 3DS/2DS system. The video below shows what the theme consists of and the awesome Metroid musical theme. When you boot up any game or app, it also makes the Samus missile sound efffect.
Use this link to download the theme for yourself.