MacLachlan's Sarah, Plain and Tall has quite a few themes, and the question asks for the main theme. Picking the single main theme for this book will likely differ slightly from reader to reader because each reader is approaching the novel from a unique perspective.
For me, the most prevalent theme is the theme of family. The book tells the story of the Witting family's search for a woman to fill the emptiness left by the death of Jacob's wife. The desire for his family to feel complete is strong enough to convince Jacob to put an ad in the newspaper for a new wife and mother to his children.
Sarah answers the ad, and she seems to be a perfect fit. I like this bit of text from chapter four.
Sarah brushed my hair and tied it up in back with a rose velvet ribbon she had brought from Maine. She brushed hers long and free and tied it back, too, and we stood side by side looking into the mirror. I looked taller, like Sarah, and fair and thin. And with my hair pulled back I looked a little...
(The entire section contains 2 answers and 585 words.)