userLocationMarker: Improve accuracy guard

https://bugzilla.gnome.org/show_bug.cgi?id=736572
instruction-box: Rely on standard css

The custom css for listbox seems good enough and
our custom one was creating some problems with white
on gray text.
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https://bugzilla.gnome.org/show_bug.cgi?id=736346
sidebar: Set default focus to From entry

https://bugzilla.gnome.org/show_bug.cgi?id=736569
Release 3.13.92