DevTools: [Documentation] Update empty page style

FYI: screenshot is available here: https://code.google.com/p/chromium/issues/detail?id=391593#c30

BUG=391593

Review URL: https://codereview.chromium.org/532223002

git-svn-id: svn://svn.chromium.org/blink/trunk@181721 bbb929c8-8fbe-4397-9dbb-9b2b20218538