Document.ActiveTheme (Word)

Returns the name of the active theme plus the theme formatting options for the specified document.

The ActiveTheme property returns "none" if the document doesn't have an active theme. For an explanation of the value returned by this property, see the Name argument of the ApplyTheme method. The value returned by this property may not correspond to the theme's display name. To return a theme's display name, use the ActiveThemeDisplayName property.

Sub CheckTheme() 
 ActiveDocument.ApplyTheme "artsy 100" 
 MsgBox ActiveDocument.ActiveTheme 
End Sub