So according to the lore, Witchers are supposed to have near perfect vision in the dark. But unless I am using a Cat potion, I can't see more than 30cm in front of me. So I am looking for a mod that gives Witchers the kind of nightvision the lore says they are supposed to have.

There is no lore that says Geralt, or other witchers, have perfect vision in the dark. His vision is much like a cat or dog's vision, meaning he has much better light perception in the dark. That does not mean he has magical vision that can see in pitch dark caves where there is zero light. Even night creatures like owls rely on other senses in the dark.

You might find a mod on nexusmods.

And a passage from The Lesser Evil (found in Polish so my translation -not that good- will follow). It's a dialog between Geralt and Caldemeyn, a mayor of Blaviken, and Libusze is Caldemeyn's wife.

-Where will you stop? Maybe at my place? There is a free room in the attic, why would you allow innkeepers to rip you off, those thieves. We will talk, and you will tell me what is going on in the world
-Gladly, but what will your Libusze say? Last time it could be seen that she dislikes me.
-In my house women have no voice. But between us don't do what you did during supper last time.
-Is it about throwing a fork at the rat?
-No, it's about that you hit it, although it was dark.
-I thought it would be funny.
-It was. But just don't do this in Libusze's presence.

And there are more, but this is one that I could just recall on top of my head.

Yeah I was browsing there earlier, but since there are thousands there (some with randomly odd names), I have no idea what to search for. Was hoping another player had already found one, and would name it.

Anonymised query "site:www.nexusmods.com witcher3 night vision" (w/o the quotes) yields
Toggleable Cat Potion-Nightvision Effect at The Witcher 3 Nexus -->
http://www.nexusmods.com/witcher3/mods/1677/
Noit sure if this is what you had in mind, but maybe you can use the potion-less variant as a workaround.

Some research is better done by yourself as you know more than anyone what you are looking for:-)