dotfiles/.config/gtk-3.0/widgets/notebook.css

/*
 * for menu popups, at least the headers
 */

notebook,
notebook tab:checked {
    background: @acc1_color;
    color: @bg_color;
}

notebook tab,
notebook header {
    background: @bg_color;
}

notebook tab:active {
    background-color: #ff0000;
}