Boundary styles have to be defined via "skinparam package<<...>>" (too)

[kirchsth]

If a (System_)Boundary contains no e.g. System then it has a different visual appearance, like below.

@startuml
!include https://raw.githubusercontent.com/plantuml-stdlib/C4-PlantUML/master/C4_Context.puml

System_Boundary(c1, "Sample System Boundary") {
System(twitter, "Twitter")
}

System_Boundary(c2, "Sample System2 Boundary") {
}
@enduml

The problem is that the corresponding "skinparam package" defintions are missing too.
(details see https://forum.plantuml.net/12761/rectangle-with-sub-entries-uses-different-font-style-color)