That is a very difficult question, especially at Christmas time. Depending on what type of concert or program you are doing, possibly the easiest way to choose a theme is to look for a great song title and arrangement.
There have been so many great songs written for Christmas that no matter the style of your concert or the group you are working with, a theme songs is sure to emerge that will fit the bill.
For example, if you are working with a show choir, large or small, old or young, pop or traditional, you'll be able to find many songs that can serve as the THEME. Examples may be:
"We Need a Little Christmas," "Happy Holidays," "Deck the Halls," "Be a Santa," "Rockin' Around the Christmas Tree," and so many more.
If you are doing a concert of more classical choral offerings, you may choose:
"Sing We Now of Christmas," "O Holy Night," "Gloria," or any title of the choral work and composer you are going to feature such as "Handel's "Messiah."
If you are putting together a program for a Church meeting, you might choose titles of traditional CAROLS or sacred songs like:
"Joy to the World," "Come to the Manger," "O Come All Ye Faithful," "O Come, O Come Emmanuel," and so many others.
"Ring Glad Christmas Bells," "O Tidings of Comfort and Joy," "On a Sacred, Still and Silent Night," "Lord of All." "The Greatest Christmas Card in the Whole Wide World," "Under the Christmas Tree," "The Christmas Alphabet," or "Don't Wait 'til the Night Before Christmas to Be Good"