Yeah (I know, windows…). I will try this in the mockup.
I spend some time in Ubuntu 17.10 changing the ubuntu.css to test the main colors (could not change the window decorations/frame, just don’t find it till now) and the top-bar and dock are looking good in #252525 or rgba(37, 37, 37, 0.98) and also down to rgba(37, 37, 37, 0.95), then it starts to lose focus to transparency (until Gnome implements blur). The proposed #2e2e2e is a little too much like the old grey for me and not as stylish as the darker tone.
When jumping to a maximized window the top-bar gets a little darker and no transparency anymore to rgba(25, 25, 25, 1.0), the transition is soft and cool by setting it to 1000ms in both cases. And it feels a little like a more content focus mode. I really like it! Also fun just to play around with windows now to see the sleek transition^^
But like always this has to be tested on different screens to find the right grey (always test mockups in fullscreen!) and of course the window decorations/frame still have the wrong colors and the dock is not changing color with the top-bar and I could not test box-shadow for the top-bar in standard mode, but in maximized window it works…