Gmail custom themes not working in Safari

Starting yesterday afternoon (2/16/17), custom themes in Gmail started showing as a dark gray background instead of the selected photo. This is only an issue in Safari -- not in Chrome or Firefox, where the custom themes still show up accurately.

Based on the responses to this issue in this Google Support Forum, many others are having identical an identical issue that cannot be resolved independently, and it is not a Gmail problem -- it appears to be a Safari problem. Please advise as to how to resolve this. Thanks!