Flat look actually just changes the styling of UI elements in Haiku - nothing much changes apart from that. It’s basically just the current UI with a modern feel to it. The person who made it seems to have thought it out really well and has gathered feedback on the forums, and you can see screenshots for yourself here:
I should note that there is a clear distinction between what is a button and what is not, so even though it has a “flat” look the buttons still look slightly raised.