Thief: Why are the edges of my screen darkened?

Thief: Why are the edges of my screen darkened? - Crop hacker silhouette typing on computer keyboard while hacking system

So, I picked up an inexpensive copy of Thief and finally getting around to playing it. While I'm playing, the edges and corners of my screen have a darkened overlay on them. Why does the game do this? Does this provide some meaning to me? Am I sick? Am I poisoned? Is this a game mechanic?

If it doesn't mean anything, how do I get rid of it?

monitor thief

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Thief: Why are the edges of my screen darkened? - Blue billboard saying Love is why placed on road surrounded by cars and street lights against black night

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Sources: Stack Exchange - This article follows the attribution requirements of Stack Exchange and is licensed under CC BY-SA 3.0.

Images: Anete Lusina, MOHI SYED, Pixabay, Kelly