Night mode: plantuml style

[martinthomson]

When night mode is enabled, plantuml diagrams become quite hard to read as the styling used for the diagram does not change. Black on dark grey is a little challenging even for me.

[WillGoGithub]

My temporary solution is add CSS style and Html.div in every note.

<style>
    .uml {
        background-color: whitesmoke;
    }
</style>

<div class="uml">
 
\```plantuml
@startuml
.
.
@enduml   
\```

</div>