Tweak service nav tab's background.

Set the background to something that blends with the canvas, but won't
allow canvas elements (relations lines, services, etc.) to bleed
through.
Merge pull request #559 from kadams54/tab-fix

Tweak service nav tab's background.

Set the background to something that blends with the canvas, but won't
allow canvas elements (relations lines, services, etc.) to bleed
through.