paper-gtk-theme/Paper/gtk-3.0/widgets/expander.css

/************
 * expander *
 ************/
.expander {
	border: alpha(@foreground, 0.7);
    color: alpha(@foreground, 0.7);
}

.expander:active,
.expander:hover {
	border: alpha(white, 0.7);
    color: alpha(white, 0.7);
}

.expander:insensitive {
	border: alpha(@insensitive_text, 0.7);
    color: alpha(@insensitive_text, 0.7);
}

.expander:backdrop {
	border: alpha(@backdrop_text, 0.7);
    color: alpha(@backdrop_text, 0.7);
}


/* Labels */
.expander GtkLabel {
    color: @text;
}

.expander:focus GtkLabel {
    /*color: shade(@text, 2.0);*/
    color: @text;
}

.expander:active GtkLabel,
.expander:hover GtkLabel {
    color: alpha(#000, 0.5);
    /*color: @text;*/
}

.expander:backdrop GtkLabel,
.expander:backdrop:hover GtkLabel,
.expander:active:backdrop GtkLabel {
    color: @backdrop_text;
}

.expander:insensitive GtkLabel {
    color: @insensitive_text;
}

.expander:insensitive:backdrop GtkLabel {
    color: mix(@backdrop_text, @insensitive_text, 0.5);
}