Looks like I need to fix this in the next update. In the meantime, if you use Jetpack, you can use the Edit CSS feature to create custom CSS, or if you are not using it, get a plugin called Simple Custom CSS and add this as a temporary fix:
.custom-background .sidebar:before {
background-color: #fff;
}
Let me know if that works for you.
Thanks for that, I put the code in the style.css of my child theme and it works.
You’re using a child theme…even better. Anyway, you have it working, so I will set this topic as resolved. I made a note to add this fix to the next Seasonal update.