I dislike the moving around with middle mouse button and yes, the Marble is too bright. The Cyan in general is a bit of a problem, besides the obvious green on blue problem I also think it makes it really counterintuitive when suing for peace if an item is checked or not, idk.
They should also check the diplomacy view again and integrate diplomacy into the macro builder, seriously, this is such a great QOL feature in EU4, just being able to check who would accept an alliance etc.
There's also some weird aesthetic elements imo (like the view of the city at the top of every country selection screen - why? We previously saw the traditions there, which we now don't see at all on startup) or weird spacing / decisions on what information to provide (like a big part in the diplomacy view is the ship count and military level for some reason, but not troop count, also that would probably fit in a single line)
I'd also really like the process of raising Levies to be easier. I get it that they can't use the big "Raise all" button you get in CK3 because raising all of them at once isn't necessarily desired, but either going to the Military menu (which might be open on a different tab) or having to go to the Levies mapmode seems a bit much for something you want to do as quickly as possible at the start of a war.
I'd still consider it an improvement overall.