custom_functions.rb

module Sass::Script::Functions
  def user_color
    sass_user_color = Sass::Script::String.new(options[:custom][:user].color, :string)
    assert_type sass_user_color, :String
    Sass::Script::String.new("##{sass_user_color.value}")
  end
end

routes.rb

map.stylesheet "/stylesheets/*path", :controller => "runtime_stylesheets", :action => "show"

runtime_stylesheets_controller.rb

# Note: This might not work at all -- I didn't run it or anything.
class RuntimeStylesheetsController < ApplicationController

  before_filter :lookup_user
  caches_action :show, :cache_path => :show, :ttl => 24.hours
  
  def show
    contents = File.read("#{Rails.root}/app/runtime_stylesheets/#{params[:path].join("/")}")
    render :text => Sass::Engine.new(contents, :custom => {:user => @user}).render, :content_type => "text/css"
  end

  private

  def lookup_user
    @user = User.find_by_id(session[:user_id])
    render :text => "", :status => :not_found unless @user
  end

  def show_url
    "user_stylesheets/#{@user.id}/#{params[:path].join("/")}"
  end

end

stylesheet_usage.scss

body {
  background-color: user-color();
}