build(checkboxes): add indeterminate style

Author: wuda-io

components/checkbox/_checkboxes.scss

@@ -58,6 +58,11 @@ input[type="checkbox"]:checked:after {
   clip-path: polygon(0 48%, 0 0, 56% 0, 74% 17%, 40% 64%, 23% 50%, 14% 61%, 42% 82%, 81% 31%, 32% 0, 100% 0, 100% 100%, 0 100%, 0 49%);
 }
 
+[type="checkbox"]:indeterminate:after {
+  background-color: var(--md-sys-color-primary);
+  clip-path: polygon(0% 0%, 0% 100%, 18% 100%, 16% 43%, 83% 42%, 83% 63%, 16% 63%, 16% 100%, 100% 100%, 100% 0%);
+}
+
 // Disabled
 input[type="checkbox"]:disabled:before {
   filter: opacity(0.2);