Fix dark theme diff box header backround color (#4674)

public/less/themes/arc-green.less

@@ -760,8 +760,15 @@
  .CodeMirror {
      color: #9daccc;
      background-color: #2b2b2b;
-     border-top: none;    
+     border-top: none;
 }
  .CodeMirror-gutters{
      background-color: #2b2b2b;
 }
+
+.repository .diff-detail-box {
+    background-color: inherit;
+    .detail-files {
+        background-color: inherit;
+    }
+}