#400 support new visual style for C4 diagrams with !NEW_C4_STYLE=1

[kirchsth]

fixes #400

In July 2025, https://c4model.com/ introduced a new visual style for C4 diagrams. C4-PlantUML now supports this updated styling too.

To enable the new style, use one of these options:

• Add !NEW_C4_STYLE=1 before including any C4_* files
• Use the command line argument -DNEW_C4_STYLE=1

Warning

The !NEW_C4_STYLE=1 statement must be placed BEFORE all C4_ !include statements.

The new style can be combined with existing themes and customer specific tags.

It can be tested via my extended branch like below:

@startuml
' activates new style. It has to be set before a theme or a C4_* file is included
!NEW_C4_STYLE=1
 
!include https://raw.githubusercontent.com/kirchsth/C4-PlantUML/extended/C4_Container.puml

LAYOUT_LANDSCAPE()

Person(admin, "Administrator")
System_Boundary(c1, 'Sample') {
    Container(web_app, "Web Application", "C#, ASP.NET Core 2.1 MVC", "Allows users to compare multiple Twitter timelines")
}
System(twitter, "Twitter")

Rel(admin, web_app, "Uses", "HTTPS")
Rel(web_app, twitter, "Gets tweets from", "HTTPS")

SHOW_LEGEND()
@enduml

If the image is not correct displayed please refresh the page or open the link.

Following with theme and additional tags,...

BR Helmut

[Potherca]

Great work!