gistfile1.rb

module Sass::Script
  module Functions
    def base_color
      Sass::Script.parse($PICKER_BASE_COLOR, 1, 1, "PICKER_BASE_COLOR")
    end
  end
end

gistfile2.rb

get '/demo/style/:name' do
  $PICKER_BASE_COLOR = params['base_color']

  content_type 'text/css', :charset => 'utf-8'
  sass :"stylesheets/demo/#{params[:name]}", Compass.sass_engine_options
end