override variables of checkbox

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

@@ -103,3 +103,8 @@ $progress-height: 4px;
 $progress-border-radius: 0;
 $progress-bg: #f0f0f0;
 $progress-box-shadow: none;
+
+//== Custom Checkbox
+$custom-checkbox-indicator-border-radius: 0px;
+$custom-checkbox-indicator-indeterminate-box-shadow: none;
+$custom-control-indicator-box-shadow: none;