Yeah, if you replace the CSS for your custom layout with HTTPS versions of the images, the mixed content warning will go away. (Mood themes, too, if you have a custom one.) I've opened an issue to see if there's any way we can proxy HTTP images in CSS and mood themes, but there might not be; if not, you'll have to find a place on the internet that will host them for you using HTTPS. (Or even put them on DW.)