bug#51404: Support system dark mode on Windows 10

[Eli Zaretskii]

> Date: Tue, 26 Oct 2021 17:01:51 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: 51404@debbugs.gnu.org
> 
> Thanks.  I have some comments and questions below, but in any case
> these changes are large enough to require copyright assignment from
> you.  If you'd be willing to start the legal paperwork at this time, I
> will send you the form to fill with the appropriate instructions.

Actually, I now see that you already started the legal paperwork
rolling, so we are okay in that department.

Thanks.