gild the lily, to add unnecessary ornamentation, a special feature, etc., in an attempt to improve something that is already complete, satisfactory, or ideal: After that wonderful meal, serving a fancy dessert would be gilding the lily.
Origin of gild
1300–50;Middle Englishgilden,Old English-gyldan; akin to gold
Add unnecessary adornment or supposed improvement. For example, Offering three different desserts after that elaborate meal would be gilding the lily. This expression is a condensation of Shakespeare's metaphor in King John (4:2): “To gild refined gold, to paint the lily ... is wasteful and ridiculous excess.” [c. 1800]