Thanks for your request. If you try to save _message.HtmlBody as separate HTML file and open this HTML document using any browser, you will see, there is no any background. After inserting this HTML inside a Document, you will see the same result.
Thanks for your request. If you have an ability to get this background using Aspose.Network then you will be able to set this background using BackgroundShape property of Asose.Words document object. Please see the following link: