I am getting error on it saying it can't be set declaratively, but how do I set it? What did this change to?
Border.Thickness is the equivalent of the BorderWidth property.
BorderWidth as well as BorderColor were removed from the serializable properties in order to fix a problem where those settings conflicted with the settings under the Border property. For many existing apps, this change resulted in the error you encountered because BorderWidth and BorderColor had already been serialized to the .aspx page.