in a class diagram, I tried to change the thickness of borders for notes using skinparam, but the thickness remains unchanged. I could change the color of the border successfully.
skinparam NoteBorderThickness 10
skinparam NoteBorderColor green
note as node1
This is a test note!
I tried it with the last beta of plantuml and the online application (http://plantuml.com/plantuml/png/ROkn3O0m30HxJ_5iG5Q0dee5B6915B2HdVr5e4RwudL_7xNepSuN5ciocMVnzIXRLOb06Yb-XDbES-mkeaJQCJYWbYKHqD-17aQJQ7ZdWKJprnu0), with the same result in both cases.