add style file

src/client/styles/scss/_override-handsontable.scss

@@ -0,0 +1,3 @@
+.modal .handsontable .wtBorder {
+  z-index: 110;
+}

src/client/styles/scss/style.scss

@@ -14,6 +14,9 @@
 // override react-bootstrap-typeahead styles
 @import 'override-rbt';
 
+// override Handsontable styles
+@import 'override-handsontable';
+
 // crowi component
 @import 'admin';
 @import 'attachments';