toString({attached, ...options} = {}) {
    const {linebreak} = getWhitespaceSymbols(options)
    let css = ''
    for (let i = 0; i < this.registry.length; i++) {
      const sheet = this.registry[i]
      if (attached != null && sheet.attached !== attached) {
        continue
      }
      if (css) css += linebreak
      css += sheet.toString(options)
    }
    return css
  }
Convert all attached sheets to a CSS string.