application_controller.rb

class ApplicationController < ActionController::Base
  protect_from_forgery

  private
  
  # Define theme of user
  def define_theme
    if @site.theme
      @site.theme
    else
      "default"
    end
  end
  
end

news_controller.rb

class NewsController < ApplicationController
  theme :define_theme 

  # .. actions bellow
end