This is the correct answer. There is an image in the custom CSS that comes from a site with an expired security certificate or other issue. This question comes up fairly often and causes suspicion when the reality is that Voat is not the cause of the warning. CSS creators don't seem to think it is a problem to use content from other sites but there is always a risk when doing so. I have custom CSS turned off and don't ever see this warning on Voat.