In your sample document, there is an extra level of complexity - you tables are contained inside text boxes. Try this code
Code:
Sub ApplyTableStyle()
Dim t As Table, aShp As Shape
For Each t In ActiveDocument.Tables
t.Style = "Light Shading - Accent 3"
Next
For Each aShp In ActiveDocument.Shapes
For Each t In aShp.TextFrame.TextRange.Tables
t.Style = "Light Shading - Accent 3"
Next t
Next aShp
End Sub