fdo#70159 Comments label: Use darker color when not highlighted

See screenshots 'before' and 'after' applying this patch in bug report.

The contrast was to low when the comments ruler button isn't highlighted
Make use of the DarkShadowColor, which is as far I can see also used
for the ruler of the document.

Change-Id: I730f9a8320cbd424c14f4cb0dc68b7f14c1df0b2
Reviewed-on: https://gerrit.libreoffice.org/7768
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>