add variable for custom-checkbox design

src/client/styles/scss/_override-bootstrap-variables.scss

@@ -107,3 +107,4 @@ $progress-box-shadow: none;
 //== Custom Checkbox
 $custom-checkbox-indicator-border-radius: 0px;
 $custom-control-indicator-focus-box-shadow: none;
+$custom-control-indicator-size: 1.2rem;