I think so. "Both are nice" is correct and there is a same word in negative form that it is " Neither ". you can use both forms using "Neither" like: Neither of them is red. ( here we have 2 pens. one of them is black and the other one is blue) Neither of them are red. ( " " " )

"Both" refers to two things, so your only option is plural: "Both are..."

Both are nice . "Both" is plural, so the verb must also be plural.

I think it should be plural. Both are nice. -Both +plural verb -Both(the)+plural noun+plural vervb -Both of +plural noun +plural verb