それにしても details
Examples
even so — the previous statement is granted, and the speaker's reaction to it holds anyway
忙しいのは分かるよ。それにしても、連絡ぐらいしてほしかったな。
isogashii no wa wakaru yo. sore ni shite mo, renraku gurai shite hoshikatta na.
I get that you're busy. Still, I wish you'd at least been in touch. casual
値上げは仕方がないでしょう。それにしても、二倍は高すぎます。
neage wa shikata ga nai deshou. sore ni shite mo, nibai wa takasugimasu.
A price rise may be unavoidable. Even so, double is too much. neutral
それにしても、今日は暑いね。
sore ni shite mo, kyou wa atsui ne.
My word, it's hot today. casual
それにしましても、本日はお足元の悪い中、よくお集まりくださいました。
sore ni shimashite mo, honjitsu wa o ashimoto no warui naka, yoku o atsumari kudasaimashita.
All the same, how good of you all to come out in weather like this. formal
Formation
それにしても、clauseattaches to plain clause
Notes
takes the situation everyone is standing in as what it grants, which is why
the connective can open a conversation.
Related patterns
Similar
にしても sits inside its sentence on a noun or a clause and grants that one case out of several (子供にしても大人にしても); それにしても takes the whole preceding sentence as what it grants, which is why it can also open with nothing granted at all (それにしても暑いね).
とはいえ concedes a point in order to turn against it, so the clause after it must run counter to expectation, and it leans written; それにしても needs no reversal — characteristically it amplifies what was already said (それにしてもよく降るね) and it belongs to speech.
Lookalikes
One particle apart, opposite verdicts — それにしては measures what follows against the premise and reports a MISMATCH, それにしても accepts the premise and asserts what holds anyway; read one for the other and the sentence's conclusion inverts.