...she thought of his regard with a deeper sentiment of gratitude than it had ever raised before; she remembered its warmth, and softened its impropriety of expression. (Ch. 43)

Does "impropriety of expression" mean the way Darcy "expressed" his love for her in his proposal? I had previously interpreted expression as the way he looked in the portrait (which confused me); I think I had that wrong and had missed the link between "impropriety of expression" and the preceding phrase "thought of his regard" which might specifically refer back to the first proposal rather than being a more general reference.