The Diagram is rendered using vector graphics via the Drawing library
The actual rendering target can be Canvas, SVG, VML or PDF.
Diagram shapes are missing CSS classes as we can't support them over the whole range of outputs.
What we use instead is declarative styling via the widget properties.
are also styled via code.
I hope this sheds some light on why it's not possible, or at least not advisable, to use CSS styling.
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