bug#51404: Support system dark mode on Windows 10

[Eli Zaretskii]

> Date: Thu, 28 Oct 2021 10:15:40 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: 51404@debbugs.gnu.org
> 
> > From: Vince Salvino <salvino@coderedcorp.com>
> > CC: "51404@debbugs.gnu.org" <51404@debbugs.gnu.org>
> > Date: Wed, 27 Oct 2021 21:41:05 +0000
> > 
> > Update: I managed to get my hands on an 1809 system and was able to get 
> > dark mode working there as well. As far as I can tell 1809 is the absolute 
> > minimum as that is when this setting and dark mode Explorer were introduced 
> > into Windows.
> > 
> > The advantage is that this will now work on Windows Server 2019 and Windows 
> > LTSC 2019, which some folks may be limited to as those are the latest 
> > Server and LTSC releases.
> > 
> > Attached patch includes the complete change, with relevant notes etc.
> 
> Thanks.  Your legal paperwork also came through, so I will be
> installing this soon.

Now done, with a few minor adaptations to our style conventions.

Please in the future accompany your changes with ChangeLog-style
commit log messages, as described in CONTRIBUTE.  (I added those for
you in this case.)

Can we now please implement the Emacs-specific user setting that will
allow users to opt in or out of this feature?  Here's what I suggest:

  . define a variable exposed to Lisp using DEFVAR_BOOL; let's call it
    w32-follow-system-theme
  . move the determination of w32_darkmode from globals_of_w32fns to
    w32_term_init, and make it depend on the value of
    w32-follow-system-theme: only set w32_darkmode if the variable is
    non-zero
  . document that users can customize w32-follow-system-theme in their
    early-init file (which is processed before window-system
    initialization that calls x-open-connection)

WDYT?

(Let me know if you need help in making any of the above happen.)

Thanks.