Hi,
We do not have any options that explicitly turn on or off background color. So it must be from your CSS style. Another thing you can check is if you have HtmlToPdf.Options.UsePrintMedia set to true, then it will use your print media CSS rules.
If you still can not find what triggered it, you can try to isolate the problem into a separate test project and send the test project to us:
https://www.essentialobjects.com/forum/test_project.aspxOnce we have that we will look into it and should be able to find out the root cause for you.
Thanks!