Refactor: extract method, constants
Change menu button background color to transparent, reduce border radius.

[ci skip]
Style the main menu

[skip ci]
Change main menu to white background

[ci skip]
Change main menu bar to 'position: fixed;'

Also needed to set it's z-index to -1 and to replace

  click_link 'sign out'

with

  find('a', text: 'sign out').trigger('click')

to work around an ongoing possible issue in Capybara/Poltergeist/PhantomJS.
See https://github.com/teampoltergeist/poltergeist/issues/60