promotion-teacup.rb

Teacup::Stylesheet.new :main do
  style :my_style,
    constraints: [
      constrain_left(0),
      constrain_width(100),
      constrain_top(0),
      constrain(:bottom).equals(:superview, :bottom),
    ]
end

class HomeScreen < PM::Screen
  stylesheet :main

  def will_appear
    add UIView, :my_style
  end
end

Nevermind, I just had to call this way:
add UIView, { stylename: :background }