According to which theory is an acid defined as an electron pair acceptor?

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The main idea is the Lewis concept: an acid is any species that accepts an electron pair, while a base donates an electron pair. This broad view lets us describe acid–base chemistry without involving protons. For example, boron trifluoride accepts a lone pair from ammonia, forming a coordinate covalent bond in the adduct, which is precisely how a Lewis acid–base interaction works. The emphasis is on electron-pair transfer, not hydrogen ions. By contrast, Bronsted-Lowry treats acids as proton donors, and Arrhenius defines acids by increasing H+ concentration in water; both rely on proton transfer. Quantum theory isn’t a standalone acid–base definition, but rather a framework for understanding molecular orbitals. So defining an acid as an electron-pair acceptor fits the Lewis theory.