Add outline to :focus-visible state for input.filter__checkbox to ensure visibility in Safari.

Closes #3365103

css/pb.css

@@ -28,6 +28,10 @@
 .filter__checkbox {
   margin-inline: 10px;
 }
+input.filter__checkbox:focus-visible {
+  outline: 3px solid #26a769;
+  outline-offset: 2px;
+}
 .filter__fieldset {
   border: none;
 }