ensured that the styling in diff.css gets imported into the bundle

Author: telamonian

style/diff.css

@@ -1,3 +1,8 @@
+/*-----------------------------------------------------------------------------
+| Copyright (c) Jupyter Development Team.
+| Distributed under the terms of the Modified BSD License.
+|----------------------------------------------------------------------------*/
+
 .jp-git-Notebook-diff .CodeMirror-gutter.CodeMirror-linenumbers[style] {
     width: 29px !important;
 }
@@ -506,4 +511,4 @@
     -ms-transform: translate(0, -100%);
     transform: translate(0, -100%);
     height: 0;
-}
+}

style/index.css

@@ -1 +1,7 @@
+/*-----------------------------------------------------------------------------
+| Copyright (c) Jupyter Development Team.
+| Distributed under the terms of the Modified BSD License.
+|----------------------------------------------------------------------------*/
+
+@import 'diff.css';
 @import 'variables.css';

style/variables.css

@@ -2,6 +2,7 @@
 | Copyright (c) Jupyter Development Team.
 | Distributed under the terms of the Modified BSD License.
 |----------------------------------------------------------------------------*/
+
 @import url('https://fonts.googleapis.com/css?family=Oswald');
 
 :root {