Fix some test snapshots and css

packages/application/style/base.css

@@ -80,6 +80,3 @@ body[data-notebook='notebooks'] #main-panel {
 body[data-notebook='notebooks'] #spacer-widget {
   min-height: unset;
 }
-
-/* Sibling imports */
-@import './sidepanel.css';

packages/application/style/index.css

@@ -8,3 +8,4 @@
 @import url('~@jupyterlab/ui-components/style/index.css');
 
 @import url('./base.css');
+@import url('./sidepanel.css');

packages/application/style/index.js

@@ -8,3 +8,4 @@ import '@jupyterlab/mainmenu/style/index.js';
 import '@jupyterlab/ui-components/style/index.js';
 
 import './base.css';
+import './sidepanel.css';