RE: Dark mode on Windows

[Vince Salvino]

> For instance, Emacs could define a system-independent way to specify "dark 
> mode".  Perhaps using an environment variable.  I'd expect that to work on 
> all systems.  If a system has a conventional way to specify this. then Emacs 
> should recognize it.

I agree this would be a fun feature, so that users could then customize their 
own configuration around the system variable. However this particular patch 
does not add any new features, it is simply making the GUI on MS-Windows more 
consistent with the other supported window managers. Users of free operating 
systems already enjoy this feature :)  For example, in Emacs 27.2:

* GTK: Changing the GTK theme (e.g. adwaita to adwaita-dark): the title bar of 
Emacs GUI is now dark, following the GTK theme (controlled by window manager).

* macOS: changing the system preference from light to dark: the title bar of 
Emacs GUI is now dark, following the macOS system preference (controlled by 
window manager).

* MS-Windows: changing the system preference from light to dark: the title bar 
of Emacs GUI remains white, even though the system theme is dark (the window 
manager requires opt-in from the program to respect the system preference). 
This patch fixes that behavior by providing the necessary opt-in to respect the 
system setting.

I will try to make a second patch following your advice, independent of this 
patch, which exposes the system light/dark preference to Emacs, via a variable, 
so that the user can read or write it at their pleasure. This is going to 
require knowledge of all the various window managers supported by Emacs, so 
help will be appreciated!

Vince Salvino