I'm glad this wasn't just my dashboard. Probably a bug
Viewing post in Why is my dashboard in greyscale?
The bug as such might be an accidental rollout of a test version of the site or whatever.
But the thing itself is kinda very specific and done in a way not consistent to the other applications of css on Itch. All the Itch css is in files. That grey thingy is injected in the html itself.
What worries me is lack of official response. For site issues, leafo is often quite quick to say something about it.