Start Center Template Manager Tweaks

Template Manager and the related Start Center view has same
transparent highlight color as Recent Documents. Also Template
thumbnail containers should have the same size as Recent Docs view.

Change-Id: I22e417a5c5ca5a849577d5a438a193371368fe24
Reviewed-on: https://gerrit.libreoffice.org/11467
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>