Nothing really specific about tweening a property here, pretty standard stuff:
tween.tween_property(self, "theme_override_colors/font_color", Color(1, 1, 1, 1), 5)
(edit: this will change the colour to white over 5 seconds)
You could also tween modulate too. Property names can be found in the Inspector tooltips (can also copy them from here too, paste into code).