I really like the new UI (much better then the previous beta version) but in my opinion there is just too much Windows grey being used for the interface - which just looks a bit dated.
I’m not good at using Photoshop but I have made 2 different themes. I have just used the paint bucket to change the colors so the lines between the icons in the sidebar are missing, the icons are a bit messed up and there isn’t any shade in the menu bar anymore. (It is highly appreciated if someone has the skill and time to make it more professional looking)
I’d also recommend to completely remove the status bar at the bottom of the window as this also looks dated and isn’t even used in new apps like Numbers, Pages or Airmail. I’m sure there is another way to incorporate the current functions of the statusbar.
This is the dark theme (inspired by Twitter, Airmail, Photoshop CC):
This is the light theme:
This is the default grey:
and this is Windows NT: