Background Trimming (Oval) Description
An oval-shaped device with a 1/8 inch (.32 cm) blue border, 1 3/8 inches (3.49 cm) in height and 2 1/4 inches (5.72 cm) in width overall, consisting of a white  background with a blue border notched at the horizontal center line bearing a blue lightning flash bendwise throughout.

History
The background trimming was approved on 3 January 1986.

Source: The US Army Institute of Heraldry