Add semantic class and top border to editor widget

src/editorwidget/theme.css

@@ -0,0 +1,7 @@
+/*-----------------------------------------------------------------------------
+| Copyright (c) Jupyter Development Team.
+| Distributed under the terms of the Modified BSD License.
+|----------------------------------------------------------------------------*/
+.jp-EditorWidget {
+    border-top: 1px solid #E0E0E0;
+}

src/theme.css

@@ -7,5 +7,5 @@
 @import './dialog/theme.css';
 @import './filebrowser/theme.css';
 @import './terminal/theme.css';
-
+@import './editorwidget/theme.css';