One possible interpretation of the story is to see it as an illustration of the famous maxim "what goes around, comes around." Mrs. Bixby has been cheating on her husband, cynically using him just so she can enjoy an opulent lifestyle. Essentially she's treating both her husband and her marriage as if they are worthless. So there's more than a hint of poetic justice when Mrs. Bixby ends up with less than what she bargained for. The less than impressive stole that she receives from her husband is worthless compared to the fancy mink coat given to her by the Colonel.

The theme of duplicity is an important one here, though as we'd expect from Dahl, it's given something of a twist. Mrs. Bixby, as we've already seen, has been cheating on her husband, yet she still has the decency to try and cover her tracks. However, in the final scene of the story, it would appear that Mr. Bixby has been conducting an illicit liaison of his own with his secretary. We don't know for sure, of course, but the fact that she's now wearing the Colonel's luxurious gift around her shoulders, and also exudes the whiff of expensive perfume, would seem to indicate that there's something going on between her and Mr. Bixby. If that indeed is the case, then it would seem that if you're going to be duplicitous, then it isn't always best to be discreet about it.

The major theme of this story is betrayal and deception.  The story is meant to illustrate the idea that "turnabout is fair play" when we start to deceive and betray one another.

Mrs. Bixby has been carrying on an affair for years behind her husband's back.  She has been betraying him.  Now she has gotten this mink coat from her lover.  She has to deceive her husband in some way that will allow her to keep the coat without his knowing that she has been having this affair.  But Mrs. Bixby has the tables turned on her and is deceived by her husband, who gives the coat to his assistant instead.

Mrs. Bixby has been deceived and betrayed by her husband just as she is trying to deceive and betray him.  She has gotten a "taste of her own medicine."  This is the major theme of the story.