As the question states I want that default interface back which was in Samsung default keyboard on SM-N900. Will customize rest.
I have not found that theme with swiftkey yet., only if SwiftKey releases a theme that looks just like the Samsung keyboard, you can't really change it.
Nope, you haven't found it because they haven't made one. Unfortunately (and this is a BIG unfortunately for me), SwiftKey decided to make its profit from themes, so it never offered and API for this. We're limited to whatever sloppy themes (honestly, most of them are just images slapped in the background without thinking how that'll make the keys look) they come up with and sell on their store.
You can have a bottom number row and I believe on phones you can get arrows as a bottom row, but don't think they have the .com button (but they do have the common @ domains button when inputting an e-mail address).
Every keyboard has its failings. Many of us a still waiting for the perfect blend of functionality. I am with SwiftKey while I wait. I hope I don't miss it when it arrives.