ve.ce.Node: Change slug color from darker grey to lighter blue/grey

* Change border from darker grey to light grey
  (same grey as toolbar border)
* Change color fill from medium grey to light blue
  (50% opaque version of the blue in the gradient in the toolbar)

Makes it feel lighter in general, and (in MediaWiki context)
makes it less similar to the default style for <pre>.

Change-Id: I0b1cc06e900fade4a62a4befb16a411527992f7c
Merge "ve.ce.Node: Change slug color from darker grey to lighter blue/grey"