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Hello M.,
For what kind of diagram?For Sequence diagram, see:
And with style (see https://plantuml.com/en/style-evolution):
[Click to see code on PlantUML server]
For Class diagram, see, first example on the doc.:
but style seems to be KO.
For Component and deployement, see:
But it seems to be KO, here is an example (on 2020.11):
@startuml 'skinparam useBetaStyle true <style> note { FontSize 10 BackGroundColor #fff .green { BackgroundColor green } } </style> node n file f note<<green>> left of n: note that is green @enduml
If that can help,
Regards,Th.