fdo#81489: Tweak the color in 'Insert Row/Column' to increase its visibility

... and convey its meaning better.

Change-Id: I95e33c2222975762e3a909dc110e60c502e7872b
Reviewed-on: https://gerrit.libreoffice.org/10412
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>