Fix tabbed DocsUI dark mode

Change the colour of the briefcase, tabs, and zero state screen while in
night mode.

Also fix a couple of problems from the previous styles change CL
I3456d32b88dbdcb6023d13165daecbf87419482b: fix LTR and get the 'edge'
colours the correct way around.

Fixes: 150286830
Bug: 149737528
Test: manual
Change-Id: Id62dddf6bf9a9792a14e9c3c3cf8b9af46683a7f