[Feature Request] Change PlantUML theme

[zeerd]

Is your feature request related to a problem? Please describe.
No.

Describe the solution you'd like
The plantuml is too white/light. I want it be dark or transparent.

Describe alternatives you've considered
I want to change the theme of Plantuml when using dark theme by given an user-defined-theme-file-path.
If this's an upsteam feature, please let know who/which-project should I ask.

Additional context
https://plantuml.com/zh/theme