".....Is there any way to hack the Windows 10 Dark Theme so that the inactive/unslected window's title bar stays black instead of changing to a light/white color? I would like all the windows' title bars to just stay black/dark regardless of whether they are the currently selected window/app or not. I don't want to use Windowblinds to do this if at all possible-- I own Windowblinds but I find I really like the built in Windows 10 dark theme better than any of the windowblinds themes."
When I design a skin in SKS, for Windowblinds, I do not have my inactive title bar change to a lighter color, my Active, and Inactive title bars are the same color, the only difference in my Active, and Inactive title bars is the text color, my Inactive text is slightly darker....maybe I'm wrong, but maybe, there's not a dark color/style you like in Windowblind skins, like Neil said, there is "Curtains" here at Wincustomize by Stardock, that one may find a dark/black skin that will suit your need, just saying. 
