Enhanced Theme Support

[johthor]

Is your feature request related to a problem? Please describe.
After #9 is merged the theme support consists of not applying any styling to the elements to not conflict with the theme.

Describe the solution you'd like
Instead of not applying any styles when a theme is detected the library should instead react smartly to the theme and apply only matching styles.

Additionally support for standalone themes like the C4-PlantUML themes would also be nice. That would also allow to extract the current "default styles" into a Domain-Story-Theme called "Egon" 😁

Describe alternatives you've considered
C4-PlantUML also provides C4-specific themes, but those need to be used explicitly.