Correggio has made his memory immortal by the strength he has given to his figures, and by sweetening his lights and shadows, and melting them into each other. - John Dryden.
make sweeter, more pleasant, or more agreeable; "sweeten a deal" make sweeter in taste
If you sweeten food or drink, you add sugar, honey, or another sweet substance to it. He liberally sweetened his coffee The Australians fry their bananas and sweeten them with honey
If you sweeten something such as an offer or a business deal, you try to make someone want it more by improving it or by increasing the amount you are willing to pay. Kalon Group has sweetened its takeover offer for Manders