There is going to be a very long tail of applications that will
explicitly request VT SGR 40 when what they really want is to
SetConsoleTextAttribute() with a black background. Instead of making
those applications look bad (and therefore making us look bad, because
we're releasing this as an update to something that "looks good"
already), we're introducing this compatibility hack. Before the color
reckoning in #6698 + #6506, *every* color was subject to being
spontaneously and erroneously turned into the default color. Now, only
the 16-color palette value that matches the active console background
color will be destroyed.  This is not intended to be a long-term
solution. This comment will be discovered in forty years(*) time and
people will laugh at my hubris.

Removal, or final remediation, will be tracked by #6807.

*it doesn't matter when you're reading this, it will always be 40 years
from now.