Use nord3 as background color for editor notifiations

The editor color scheme provides the `NOTIFICATION_BACKGROUND` key that
allows to style the background color of notifications from the editor
that are placed right below the tab bar at the top of the editor.
Previously `nord4` was used that made the color almost unreadable since
is uses a lower contrast by default instead of pure black.
This has now been changed to use `nord3` making the actual content
readable again.

GH-52

resources/themes/nord.xml

@@ -42,7 +42,7 @@
     <option name="LINE_NUMBERS_COLOR" value="4c566a" />
     <option name="METHOD_SEPARATORS_COLOR" value="d8dee9" />
     <option name="MODIFIED_LINES_COLOR" value="ebcb8b" />
-    <option name="NOTIFICATION_BACKGROUND" value="d8dee9" />
+    <option name="NOTIFICATION_BACKGROUND" value="4c566a" />
     <option name="QUESTION_HINT" value="434c5e" />
     <option name="RECURSIVE_CALL_ATTRIBUTES" value="" />
     <option name="RIGHT_MARGIN_COLOR" value="4c566a" />