code: function theme (env) {
      let promise = env.theme(env.dest, env)
  
      if (!is.promise(promise)) {
        let type = Object.prototype.toString.call(promise)
        throw new errors.Error(`Theme didn't return a promise, got ${type}.`)
      }
  
      return promise
        .then(() => {
          let displayTheme = env.displayTheme || 'anonymous'
          env.logger.log(`Theme \`${displayTheme}\` successfully rendered.`)
        })
    }
docs: Render theme with parsed data context.
  @return {Promise}