Diagram Theme Support (#76)

JetBrains core product version 2019.2 introduced theme support (1) for
"Diagrams" (2) in order to prevent unreadable output due to "hardcoded"
color values not matching the currently active UI theme.
This commits adds the available theme keys to better match Nord's style.

References:
  (1) https://youtrack.jetbrains.com/issue/IDEA-207533
  (2) https://www.jetbrains.com/help/idea/diagrams.html

Resolves GH-75

resources/themes/nord.xml

@@ -13,6 +13,15 @@
     <option name="CARET_ROW_COLOR" value="3b4252" />
     <option name="CONSOLE_BACKGROUND_KEY" value="2e3440" />
     <option name="DELETED_LINES_COLOR" value="bf616a" />
+    <option name="DIAGRAM_ANNOTATION_EDGE" value="d08770" />
+    <option name="DIAGRAM_DEFAULT_EDGE" value="d8dee9" />
+    <option name="DIAGRAM_GENERALIZATION_EDGE" value="88c0d0" />
+    <option name="DIAGRAM_INNER_EDGE" value="8fbcbb" />
+    <option name="DIAGRAM_NODE_BACKGROUND" value="4c566a" />
+    <option name="DIAGRAM_NODE_HEADER" value="4c566a" />
+    <option name="DIAGRAM_NOTE_BACKGROUND" value="434c5e" />
+    <option name="DIAGRAM_NOTE_BORDER" value="434c5e" />
+    <option name="DIAGRAM_REALIZATION_EDGE" value="a3be8c" />
     <option name="DOCUMENTATION_COLOR" value="434c5e" />
     <option name="ERROR_HINT" value="434c5e" />
     <option name="FILESTATUS_ADDED" value="a3be8c" />