Sorry for the delay!
I just updated the drivers and tried again. The effect is still there but It's a different colour, now it just seems to become darker shade of gray. And when I do that thing in photshop again, it looks just fine.

I also just noticed that the effect also apply's to the other background modes, and not just "theme" setting. It just depends on the colour of the background. It might even be happening in the other panels as well.
When I look closely at my screen, the viewport's colour looks a little bit grainy. It's not an even colour, but a mesh of 2 or 3 colours (and no, I don't subpixels) that are just a little different than the colour it's supposed to be.
If I look in blender 2.79, it doesn't seem grainy, but the effect is still there in a different way. It's extremely subtle though, making it not a problem.
Here a picture of the backgrounds. I took a 32x32 square out of screenshots.
The first one is "theme" in 2.8, the second and third are fully white and fully black on the "viewport" setting. The fourth one is blender 2.79.
I edited the images to make it clearer where the the colours are different. As you can see, only 2.79 is a completely even colour. I don't know why though.

You can't really see it here, but when zooming in, you can see the "noise" on the first, unedited square.