Anna and Tania went shopping, but . . . . . . . . couldn't find anything . . . . . . . . liked.

Singular 'they', along with its inflected or derivative forms, them, their, theirs and themselves, is an epicene third-person pronoun. It typically occurs with an unspecified antecedent, in sentences such as: "Somebody left their umbrella in the office. Could you please let them know where they can get it?"