shite mo, shite wa
 

Hi.
I found a sentence.
"Haru ni shite wa samui. Maru de fuyu no you da."
To me, this sentence means "It's spring but it's cold. It's almost as if it is winter."
Why not use "shite mo"?

Thank you :)

Why not use kana?

Because にしては has a specific meaning of "considering" or "for." Literally, "For spring, it's cold."

And I think you're at a level now where you ought not to be using romaji, too.