Fix wrong color border on global nav logo
Fix wrong color border on global nav logo

Fixing this bug by using nexted box-shadows, plus replacing the focus
indicaor work done under admin-833 with the box-shadows too.

fixes ADMIN-893

test plan:
  - create a theme with an odd color logo background
  - load canvas and tab to give the logo at the top of the navbar focus
  > expect there to be none of the navbar's background grey peeking out
  (high contrast mode doesn't matter, since the logo is hidden)
  - tab through the global nav
  > expect the focus indicator to match kdub's design attached to ADMIN-833

Change-Id: Ib3a41464821c4af9aa0f6dadeb632082c0a2fd9b
Reviewed-on: https://gerrit.instructure.com/144766
Tested-by: Jenkins
Reviewed-by: Chris Hart <b82b40b84a2f06957ec9caf1768340bd6b929cc4@instructure.com>
QA-Review: Dan Sasaki <ae19fabf7fe19dff310008c9b6a4d2ec84d311d8@instructure.com>
Product-Review: Ed Schiebel <0ca8a83847a5889cf05cb219c0ab09f6b646243b@instructure.com>

sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,djbender/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,djbender/canvas-lms,instructure/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms