gtk-fallback.css

.menu {
    border-image: linear-gradient(to top,
                                  alpha(@border_color, 0.80),
                                  alpha(@border_color, 0.60) 33%,
                                  alpha(@border_color, 0.50) 66%,
                                  alpha(@border_color, 0.15)) 1 1 1 1/ 1px 1px 1px 1px;
}

.menubar .menu {
    border-image: linear-gradient(to top,
                                  alpha(@border_color, 0.80),
                                  alpha(@border_color, 0.60) 33%,
                                  alpha(@border_color, 0.50) 66%,
                                  transparent 99%) 1 1 1 1/ 1px 1px 1px 1px;
}