Customization Menu Icons are Missing

We have been using Styles & Layouts for a couple of months and have never been able to see the customization menu icons.

Example: Icons to indicate Width based upon Device

Instead of, say, a gear icon to indicate settings, an arrow to indicate forward or back, an X to indicate close, or an icon that looks like a mobile phone to indicate a mobile layout option, the only icon we see is a placeholder that looks like a rectangle. Every icon is missing and replaced with a rectagle.

Any ideas? We’ve tried different browsers, Macs and PCs, etc.