Vim tries to determine the default background and checks for $TERM
and even checks for the "putty" value. But unfortunately, putty by
default uses "xterm" as $TERM value and as such Vim uses a "light"
background.
So use a TermResponse autocommand to set the background for putty back
to dark.
Note: this only works on non-tiny builds and when defaults.vim is in
use.
Signed-off-by: Christian Brabandt <cb@256bit.org>