Produces the SVG output below. I can't find any trace of the "stylesheet" that I supposed to be output as an xml processing instruction. Also tried -Gstylesheet="file.css" and -Gstylesheet=file.css - the result is the same in all cases. (I also tried a statemnt like graph [stylesheet="file.css"]; but that also gave the samme result. No css in the output.

I don't know if this is a bug, or if it is supposed to work some other way.
Input:

digraph G {
stylesheet="file.css";
a;
b;
a-> b;
}