Apply filter on icon under primary background

Otherwise the icon is not visible as it has the same color as the background.

Closes #14032

sao/src/sao.less

@@ -114,7 +114,7 @@ html[theme="default"] {
     }
 }
 
-.btn-primary {
+.btn-primary, .bg-primary {
     .icon {
         filter: brightness(0) invert(1);
     }