Retro Mode is a secret graphical option that you can access by entering the famous Konami Code. The code has been used in many Konami games and many developers have included secrets in their own games when using this code. Sheepy: A Short Adventure is one such game! I've not seen this mentioned anywhere on the Internet before, so I think this is a first.

When playing the game, enter the Konami Code as follows:

Keyboard:
Up, Up, Down, Down, Left, Right, Left, Right, B, A

Xbox:
Up, Up, Down, Down, Left, Right, Left, Right, B, A

PlayStation:
Up, Up, Down, Down, Left, Right, Left, Right, O, X

This will then alter the graphics to looks more retro, with scanlines and extra bloom on lights. After any loading screens the effect will be disabled and the code will need to be entered again.

The mode can also be quickly toggled once the code has been entered by pressing the following button:

Keyboard: V
Xbox: Back or View
PlayStation: Select or Share

On the left is the normal graphical mode. On the right is the Retro Graphical mode.

I've uploaded a video if you prefer to watch.