Add slider-fill styles

Author: lukewarlow

css-forms-1/Overview.bs

@@ -841,6 +841,11 @@ input[type=checkbox][switch]::slider-track {
   inline-size: 0;
 }
 
+input[type=checkbox][switch]::slider-fill {
+  background-color: transparent;
+  border-radius: inherit;
+}
+
 ::slider-thumb {
   border-radius: 0;
   border: none;