Add fill style for checked switches

Author: lukewarlow

css-forms-1/Overview.bs

@@ -877,8 +877,15 @@ progress:indeterminate::slider-fill {
 }
 
 input[type=checkbox][switch]::slider-fill {
-  background-color: transparent;
+  background-color: color-mix(in lab, currentColor 25%, transparent);
   border-radius: inherit;
+
+  block-size: 100%;
+  inline-size: 0%;
+}
+
+input[type="checkbox"][switch]:checked::slider-fill {
+  inline-size: 100%;
 }
 
 input[type="checkbox"][switch]::slider-thumb,