Rabbit theme to keep ratio in PDF

theme.rb

set_background("black")
match(SlideElement) do |slides|
  slides.each do |slide|
    raw_page = slide.instance_variable_get(:@page).instance_variable_get(:@page)
    width, height = raw_page.size
    width_ratio = canvas.width.to_f / width.to_f
    height_ratio = canvas.height.to_f / height.to_f
    unless width_ratio == height_ratio
      if width_ratio > height_ratio
        width_margin = canvas.width - (width * height_ratio)
        slide.margin_left  = width_margin / 2
        slide.margin_right = width_margin / 2
      else
        height_margin = canvas.height - (height * width_ratio)
        slide.margin_top    = height_margin / 2
        slide.margin_bottom = height_margin / 2
      end
    end
  end
end

include_theme("pdf")