Kwanzaa, a celebration of African American culture, will be observed from Dec. 26 through Jan. 1, 2021.     A 54-year tradition, Kwanzaa is a week filled with enjoying good food, exchanging gifts and decorating with the colors red, black and green.      The National Museum of African American History & Culture, part of the Smithsonian Institution in Washington, D.C., lists seven principles for Kwanzaa.    They are: Umoja (unity); Kujichagulia...