To embroider additional flowers to an already-pretty brocade
To improve upon something that is already good
Positive
新事物不能取代旧事物，新事物只能是锦上添花 | Xīn shìwù bùnéng qǔdài jiù shìwù, xīn shìwù zhǐ néng shì jǐnshàngtiānhuā
New things can't replace old things, they can only improve upon them
[Source]
你这件衣服很漂亮,如果能够再贴上胸花,那可以说是锦上添花了 | Nǐ zhè jiàn yīfú hěn piàoliang, rúguǒ nénggòu zài tiē shàng xiōnghuā, nà kěyǐ shuō shì jǐnshàngtiānhuā le
Your outfit is beautiful. If you could add a corsage, that would just be icing on the cake
[Source]
Icing on the cake
None
This is an HSK chengyu
《祖堂集》(Zǔ Táng Jí), a work from the Táng Dynasty (唐, 618 - 907). Known in English as "Anthology of the Patriarchal Hall," it is one of the oldest existing "encounter dialogues" from Chinese Buddhist monks. The work was originally lost for centuries before a copy was discovered in Korea by Japanese scholars.
This chengyu is from a line in the work listed in the "Origin" section above