Comment 54 for bug 762349

The old fix doesn't work any more because the names of the properties have changed.

This works for me:

notebook.terminal-notebook tab {
    background-color: #a2a1a0;
    foreground-color: #ffffff;
}

notebook.terminal-notebook tab:active {
    background-color: #ffffff;
    foreground-color: #010101;
}

I've put the file in ~/.config/gtk-3.0/gtk.css. You may adapt the colors at your liking. The original definitions are in /usr/share/themes/Ambiance/gtk-3.0/apps/gnome-terminal.css