I played around to understand the 3 different colors.
For the non-current window - why not make it the same color as the non-current window from the old tab group?
Also, I wonder why do we need 3 different colors?
What is wrong with making "Active" and "Current" the same color?
I found that I could only make a tab the "current" color if I clicked on another application, like Chrome, or if I change focus to the "Preview" window. But that base/white color that is used for current is nice and easy to read, so why waste it on the "current" window when focus is on a different window and we're not really looking at it anyway? Why not make both the "Active" and "Current" the same base/white that we currently see for "current"? I don't see why it makes a difference to change the "Active" color to the "Current" color just because we moved focus to another window? If having this 3rd color takes away having a nice clear readable tab for the non-current window - which we also need to be able read easily - then I don't think it is worth it.
I also played around in Google Chrome, toggling the active/inactive tabs and looked at the difference. Google Chrome does not distinguish between "current" and "active" (I know it is a different than an editor with many tool windows, but still). The inactive tab in Chrome does not have any gradient (although it is a little darker).
I also looked at the tabs in "Beyond Compare 3", another tool I use very much. No gradient in active or inactive tabs.
In Visual Studio 2008, I had an active window and then clicked on another application, and the "active" color did not change to "current" - they were the same. Same thing happened in VS2008 when I changed focus to the "build" window from the main editor windows - the tab color did not change. I don't think it needs to for SE either?
If you free up what you currently have for the "Active" window (the blue gradient), in order to use the base/white for both Active/Current, then you could use this blue (without the gradient) for the non-current windows?