Do ☺ nominalized “ellipses” exist?
"Of all the themes in this world, this is the sweetest; […]"
I would write "the sweetest" here in lowercase despite the previous article, since the sentence already contains themes & the person reading it actually sees, i.e. recognizes, the mental connection "the sweetest theme;" reminds me a bit of ☺ the ellipses in poetry
The sentence is grammatically correct, yes.