Customizing terminal tab color in Ubuntu 17.10

You can use the following style to get tab working:

/* gnome-terminal */
@define-color terminal_bg #300a24;
@define-color bg_color_active_tab #157bb7;


notebook.terminal-notebook header.top tab,
notebook.terminal-notebook header.top tab:hover {
    background-image: -gtk-gradient (linear, left top, left bottom,
                                    from (shade (@fg_color, 0.92)),
                                    color-stop (0.60, shade (@fg_color, 0.9)),
                                    to (shade (@fg_color, 0.85)));
    color: @bg_color;
}

notebook.terminal-notebook header.top tab:checked {
    background-image: -gtk-gradient (linear, left top, left bottom,
                    from (shade (@bg_color_active_tab, 1.2)),
                    to (shade (@bg_color_active_tab, 1.12)));
    color: @bg_color;

}

This will result in: