Add dark (blue) to available color styles

This isn't intended to be used for the case studies, but it is used for landing
pages to make the common background, and we're using landing pages for 'Work'.

config/sync/field.storage.paragraph.field_style_color.yml

@@ -23,6 +23,9 @@ settings:
     -
       value: dandelion
       label: Dandelion
+    -
+      value: dark
+      label: 'Dark blue'
     -
       value: marine
       label: Marine