Add the !theme line between @startuml and @enduml:
@startuml
...
!theme black-n-white from https://raw.githubusercontent.com/SBen-IV/plantuml-black-n-white-theme/main/theme
...
@enduml
or you can download the puml-theme-black-n-white.puml file and use locally:
@startuml
...
!theme black-n-white from path/to/file
...
@enduml
This theme is for personal use and focused on object diagrams.