fdo#71748 Fix also for hicontrast and sifr themes

A follow-up of commit e239f372c6abcd8371d0a666c828bf1c31195dd4.
Didn't notice there is fill_color.png also in these themes.

Also fixed the ugly yellow color flash of paragraph color picker while
initializing the sidebar (introduced by the above commit). This color
picker shows the actual color of a paragraph, so there is no reason to
set some default color there.

Change-Id: I17eed733d1e35ec71c482e3df906523733383e96
Reviewed-on: https://gerrit.libreoffice.org/7035
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>