Slider smaller, in darkshadow color and with a tiny shadow

Change-Id: I0e5253648749fb4190c03ab458498d64dc0388c5
Reviewed-on: https://gerrit.libreoffice.org/52912Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Heiko Tietze <tietze.heiko@gmail.com>