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Not really documentation but just a list can be obtained by means of the command:
java -Djava.awt.headless=true -jar plantuml.jar -language
here there is a section skinparameter
It could be that list is not complete, e.g. I tried it with following line and found 567 skinparam e.g. incl. "RoundCorner". But e.g. the existing "DiagonalCorner" was not in the list (used in sample bewow)
BRHelmut
java -Djava.awt.headless=true -jar plantuml.1.2021.5.jar -language...;skinparameter;567...DesignedDomainStereotypeFontStyleDiagramBorderColorDiagramBorderThicknessDomainBackgroundColor...RanksepRectangleBackgroundColorRectangleBorderColorRectangleBorderThicknessRectangleFontColorRectangleFontNameRectangleFontSizeRectangleFontStyleRectangleStereotypeFontColorRectangleStereotypeFontNameRectangleStereotypeFontSizeRectangleStereotypeFontStyleRequirementBackgroundColor...RoundCorner...
"DiagonalCorner" is not listed but can be used
@startumlskinparam rectangle<<RoundCorner>> { RoundCorner 25}skinparam rectangle<<DiagonalCorner>> { DiagonalCorner 18}rectangle "rectangle"rectangle "rectangleRound" <<RoundCorner>>rectangle "rectangleDiagonal" <<DiagonalCorner>>@enduml
@startumlskinparam rectangle<<RoundCorner>> { RoundCorner 25}skinparam rectangle<<DiagonalCorner>> { DiagonalCorner 18}
rectangle "rectangle"
rectangle "rectangleRound" <<RoundCorner>>rectangle "rectangleDiagonal" <<DiagonalCorner>>@enduml