bug#51404: Support system dark mode on Windows 10

[Eli Zaretskii]

> 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.