config.js

function CSS_compute_middleware(css, next) {
  if(this.foo === 'bar'){
    css.padding = 0;
  }
  
  return next(css)
}

function CSS_async_compute_middleware(css, next) {
  if(this.bar === 'foo'){
    ajax('/api/blah').then(next)
  }else{
    return next(css)
  }
}

reveal.css_middlewares = [
  CSS_compute_middleware,
  CSS_async_compute_middleware,
]