By the help of Khurshid Alam (khurshid-alam) on launchpad bug it can be fixed by creating Gtk3 per-user:
mkdir -p ~/.config/gtk-3.0
cat > ~/.config/gtk-3.0/gtk.css << EOF
.context-menu {
font-weight: normal;
font-family: regular;
}
EOF
This results in normal look-and-feel of context menu in Pluma: