What is a themed rom?

I see in the Development section thread marked as "Themed". Some are stated as "Themed Lollipop" but are actually 4.4 Kitkat.
So are "Themed" ROMs just "Modified" ?

[REQUEST] Why not a rom stock based (4.1.2) but S6/Note 5 Themed?

I'd like a rom like this for my beloved Galaxy S2!

What's app devs? :)