Error is at the bottom of this page: Changing colors and fonts (plantuml.com)
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Hello A.,
Yes you are right. The old command `help skinparams` is no longer maintained, since Mar 19, 2023(https://github.com/plantuml/plantuml/issues/1232#issuecomment-1475182063). Then:
@startuml skinparameters @enduml