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 like her daughter. Sarah's daughter.
Obviously Anna knows that Sarah is not her biological mother, but the desire to have a mother figure in the house is strong enough for Anna to overlook genetics.
Sarah's transition to the Wittings' way of life and region of the country is not all smooth and easy, which is why Caleb and Anna are constantly worried that Sarah will one day decide to just pick up and go back home. It's not until chapter 8 that the children see a very physical sign from Jacob and Sarah that the relationship just might work out.
Papa said nothing. But he put his arm around her, and leaned over to rest his chin in her hair. I closed my eyes, suddenly remembering Mama and Papa standing that way, Mama smaller than Sarah, her hair fair against Papa's shoulder. When I opened my eyes again, it was Sarah standing there. Caleb looked at me and smiled and smiled and smiled until he could smile no more.
The above quote is important because it is the very first time that Jacob shows physical affection for Sarah. It's a moment in the story where a reader really gets the feeling that the Witting family can actually be complete once again.