Hi Marjorie,
> We are using Portal 10.5.1 and Web AppBuilder does not display the Dashboard Theme under the theme tab
This is expected behavior, because the Dashboard theme was added to Web AppBuilder in ArcGIS Online in June 2017. Portal for ArcGIS 10.5.1 was released before that date.
> Is there any way to add it?
Not in any supported way. I suggest you use Web AppBuilder for ArcGIS Developer Edition with your Portal 10.5.1 instance. It will have the Dashboard theme.
Hope this helps,
