I have seen some other questions and it looks like it is currently not possible to change the background colour/transparency of the tool windows. If I am mistaken, please correct me, but if that really is the case, I would like to leave this as a suggestion:
As you can see in the screenshot below, I use dark images as my editor background and I think it perfectly matches the New UI minimalistic approach of design an feel. The only problem is this 2-tone colour difference between the tool window on the side and the actual editor windows, which I just could not find a way to get rid of.
Since you are taking feedback for the new UI, I would highly recommend you make this possible and maybe even include some sample dark background images and patterns because most people who sees my screen for the first time, has no idea this was even possible, but agrees that is looks awesome.