message: Make carousel button color match
diff: +- button_class = sd.BUTTON_COLOR == 'blue' ? 'Button--blue' : ''
  +
  .HomeSection__content.HomeHowItWorks
  .HomeSection__cta
  h2.Type.Type--size-h2 How it works
  .HomeHowItWorks__carousel.js-home-carousel
  | &nbsp;
  .HomeSection__cta
  - a.Button.Button--size-lg.Button--border-heavy( href='/sign_up' )
  + a.Button.Button--size-lg.Button--border-heavy( class=button_class href='/sign_up' )
  | Join Are.na
  