DevTools [regression]: use theme color as a background color for toolbar items, not only text.
A follow up to 161888 with more goodness.

BUG=317873

Review URL: https://codereview.chromium.org/95513002

git-svn-id: svn://svn.chromium.org/blink/trunk@162888 bbb929c8-8fbe-4397-9dbb-9b2b20218538