Add the helper below, and you will be able to style true/false values like this:

final-result

app/helpers/application_helper.rb:

# boolean green or red
def boolean_label(value)
  case value
    when true
      # content_tag(:span, "Yes", class: "badge badge-success")
      content_tag(:span, value, class: "badge badge-success")
    when false
      content_tag(:span, value, class: "badge badge-danger")
  end
end

your view:

= boolean_label(user.confirmed?)