«ici» is an adverb, invariable and does not change agreement. In this context (c'est . . ) 'stand-alone adjective', the subject of the verb -est - is the indefinite pronoun 'ce' and 'takes masculine gender' for grammatical purposes, even if the subject noun it represents is known to be feminine. Hence the adjectives in the construct also take masculine form. Adverbs do not change.