Add style for checkbox partial-check

Author: Jeffrey Biles

src/App.vue

@@ -83,6 +83,10 @@ input[type='checkbox'] {
   padding: 10px;
 }
 
+input[type='checkbox'].partial-check {
+  background: #ABC;
+}
+
 input[type='checkbox']:checked {
   background: #679;
 }