、。
She was barefoot, but that was the custom at the time.
Literal
She [topic-は] barefoot was [but-が], that [topic-は] that-time [possessive-の] custom was.
素足 (すあし) literally means 'bare feet' — 素 carries the sense of 'bare/plain/unadorned.' The clause-connecting が here means 'but,' linking a personal observation to a cultural explanation. 当時 (とうじ) anchors the sentence in a past era, and 習慣 (しゅうかん) means 'custom' or 'habit' — contextualizing what might seem unusual by explaining it was normal for the period.