Dynamically change SASS in a multi-tenant Rails application

assets_controller.rb

# This works by injecting SASS variables before the main template. As long as they are not 
# redefined without !default later on you're good to go. 

# Includes some local in-memory caching to not recompile SASS on every request.
# I've implemented it at the client (tenant) level, but it could equally be more granular, at 
# level of some other entity in your application. 

class AssetsController < ApplicationController
  def styles
    $stylesheets ||= {}
    custom_variables = %Q{
      $primary-color: #{@client.get_theme_primary_color};
      $secondary-color: #{@client.get_theme_secondary_color};
      $text-color: #{@client.get_theme_text_color};
    }
    hash = Digest::SHA256.hexdigest(custom_variables)

    if Rails.env.development? # Efficiently determine if stylesheets have changed
      hash += `find app/assets/stylesheets -type f -print0 | sort -z | xargs -0 md5sum | md5sum`.split(' ').first
    end

    if $stylesheets[hash].present?
      css = $stylesheets[hash]
    else
      asset_source = custom_variables + File.read('app/assets/stylesheets/application.scss')
      css = Sass::Engine.new(asset_source, {
        syntax: :scss,
        cache: false,
        read_cache: false,
        style: :compressed,
        load_paths: [Rails.root.join('app', 'assets', 'stylesheets')]
      }).render
      $stylesheets[hash] = css
    end

    render body: css, content_type: 'text/css'
  end
end