The label for "Text to display" on the Xfce panel inherits its color from a GTK+3 theme.
It becomes unreadable when the label's text color from the theme and and the color of the Xfce panel background that can be tuned from the panel's preferences fall close together.

Please add an option for choosing the font color of the displayed text. Or please, give the GTK widget name to the label to allow to refer to it from a user CSS file (https://developer.gnome.org/gtk3/stable/GtkWidget.html#gtk-widget-set-name) as it has been done for panel toggle buttons of the power manager and pulseaudio plugins that are also have been shifted to GTK+3.

Thanks.