[feature-request] Change themes in the diagram text.

[BlueFalconHD]

Instead off supplying it via options, supply in the sequence EG:

title: Sequence
theme: simple
Test->Test2: Stuff
Test2-->Test: More stuff

[bramp]

Good suggestion. However, I intentionally tried to keep the diagram, and the styling separate. However, happy to have a PR if you wish to add this.