A filmic vignetting effect, togglable in the Post-Processing menu.
Just extract it to the "Addons" folder, it is enabled by default.

("Vignetting" is a method of drawing a viewers attention towards a point of interest.
This is most commonly achieved in art and film by blurring or darkening the edges of the image to make the focal point stand out more.)

This was my first attempt at basic LUA coding from scratch. I've left descriptive notes within the autorun file for anyone that would like to know the basics on how this addon works.