A new course has been released for Super Mario Maker today themed off of NES Remix, upon beating it you'll be rewarded with a Frog Mario costume, perfect for use in the SMB3 theme. Except you can't use the costumes in any other theme than SMB 1.
"A new Event Course based on NES Remix is now available in Super Mario Maker!You can download and keep this course on your console or just beat it as it is. Why was this course released today? I have no idea. But I'm happy to get a new costume for Mario.
You get this awesome Frog Mario costume if you complete it!"~Super Mario Facebook