At the top of your C4 PlantUML .puml file, you need to include the C4_Context.puml, C4_Container.puml or C4_Component.puml file found in the root of this repo. To be independent of any Internet ...

Similar to PlantUML themes supports C4-PlantUML C4_... specific themes too (sometimes based on existing PlantUML themes). Additional to the standard themes with skinparam and style definitions ...