にせよ, and にしろ are two phrases that are often translated as ‘even if (A)’, or ‘no matter if (A)’. This structure is a combination of に, and either of the imperative forms of する; せよ, or しろ. せよ tends to be more common in written material, while しろ is more common in speech. Despite the English translations, this particular use of the imperative form of する translates more accurately to ‘so it’s (A), who cares!’, as the speaker is just strongly stating that the actual condition of (A) is unimportant in the overall sentence.
When repeated, にせよ～にせよ illustrates that regardless of either ‘(A) or (B), (C) is still true’. In many cases, (A) and (B) will only be a small sample of possibilities, with the actual meaning being ‘in (A), (B), or any similar situation at all, (C) is still true’.
にせよ and にしろ may appear following verbs, い-Adjectives, な-Adjectives or nouns in any of their standard forms.

• このイベントに参加（さんか）するにせよしないにせよ、私（わたし）に連絡（れんらく）をください。
  Whether or not you plan to attend this event, please contact me.
• 給料（きゅうりょう）が多（おお）いにしろ少（すく）ないにしろ、給料（きゅうりょう）をもらっている以上（いじょう）ちゃんと働（はたら）かないといけない。
  As long as you are getting paid, you must properly do your job whether you get paid well or not.
• 勉強（べんきょう）が好（す）きにせよ嫌（きら）いにせよ、子供（こども）である以上（いじょう）、勉強（べんきょう）はしなくてはいけません。
  Whether you like studying or not, as long as you are a child, you have to study.
• 社長（しゃちょう）にしろアルバイトにしろ、工場内（こうじょうない）での喫煙（きつえん）は禁止（きんし）されている。
  Whether you are the president or a part-time worker, smoking is prohibited in the factory.