Comment 4 for bug 1557981

Digged little bit into gtk3 css-ing and created workaround:

Add to ~/.config/gtk-3.0/gtk.css:

.nautilus-window notebook tab:checked {
    background-image: linear-gradient(to top, @selected_bg_color, alpha(@bg_color, 0) 3px);
}

Active tab will be underlined with theme color. Remove ".nautilus-window" to affect any tabs, not just content tabs in Nautilus window.

Works in 18.04.