You are saving HTML to a stream and requesting the CSS style sheet to be written into a separate file. In this case you must specify path and name of the CSS file.
// Create a new Document object.
Document document = new Document();
// Load the Word document from the specified file path.
document.LoadFromFile(@"..\..\..\..\..\..\..\Data\ToHtmlTemplate.docx");
// Set the file name for the CSS style sheet that will be used in the HTML export.
document.HtmlExportOptions.CssStyleSheetFileName = "sample.css";
// Specify that the CSS style sheet should be external.
document.HtmlExportOptions.CssStyleSheetType = CssStyleSheetType.External;
// Disable embedding images in the HTML output.
document.HtmlExportOptions.ImageEmbedded = false;
// Set the path where the exported HTML file will look for image resources.
document.HtmlExportOptions.ImagesPath = "Images";
// Treat text input form fields as plain text instead of form fields.
document.HtmlExportOptions.IsTextInputFormFieldAsText = true;
// Save the document as an HTML file with the specified file name and format.
document.SaveToFile("Sample.html", FileFormat.Html);
// Release the resources associated with the Document object.
document.Dispose();
document.HtmlExportOptions.CssStyleSheetType = CssStyleSheetType.Internal;