modify simplebar styles

packages/app/src/styles/_override-simplebar.scss

@@ -0,0 +1,3 @@
+.simplebar-scrollbar::before {
+  background-color: #666;
+}

packages/app/src/styles/style-app.scss

@@ -16,6 +16,9 @@
 // override react-bootstrap-typeahead styles
 @import 'override-rbt';
 
+// override simplebar-react styles
+@import 'override-simplebar';
+
 // atoms
 @import 'atoms/buttons';
 @import 'atoms/code';