Support background color styling of "diff" UI separator (#132)

The "diff" UI [1] renders a line to separate the different sections of a
patch/diff that was added to the IDE platform code in
JetBrains/intellij-community@f8de2a5.
In order to style the elements the `DIFF_SEPARATORS_BACKGROUND` editor
scheme key has been added to use `nord3` as background color.

[1]: https://www.jetbrains.com/help/idea/comparing-files-and-folders.html
[2]: https://github.com/Tom1206

Reported with proposed solution by @Tom1206 [2] in
#120 (comment)
Related to GH-120
Closes GH-128

resources/themes/nord.xml

@@ -22,6 +22,7 @@
     <option name="DIAGRAM_NOTE_BACKGROUND" value="434c5e" />
     <option name="DIAGRAM_NOTE_BORDER" value="434c5e" />
     <option name="DIAGRAM_REALIZATION_EDGE" value="a3be8c" />
+    <option name="DIFF_SEPARATORS_BACKGROUND" value="4c566a" />
     <option name="DOCUMENTATION_COLOR" value="3b4252" />
     <option name="ERROR_HINT" value="434c5e" />
     <option name="FILESTATUS_ADDED" value="a3be8c" />