Comment 2 for bug 1639443

Found the culprit. In Radiance's gtk-3.20/gtk-widgets.css I commented out the border-image-source under .primary-toolbar button:disabled:

.primary-toolbar button:disabled,
headerbar button:disabled {
    background-image: -gtk-gradient (linear, left top, left bottom,
                                     from (shade (@dark_bg_color, 1.01)),
                                     to (shade (@dark_bg_color, 0.99)));
    /*border-image-source: -gtk-scaled(url("assets/button-toolbar.png"),
                                     url("assets/button-toolbar@2.png"));*/
}

Attached is the modified file.