。
She had warmed up the dining room for winter.
Literal
She [topic-は] winter [in-に] dining-room [object-を] warm made-in-advance.
Two patterns combine: ~くする (暖かくする, 'make warm') turns the i-adjective 暖かい into a verb phrase, and ~ておく (しておいた, 'did in advance / set up for later benefit') adds the nuance of preparation. She didn't just warm the room — she prepared it warm, implying foresight. 食堂 can mean 'dining room' or 'cafeteria' depending on context.