You can add a line break to generated content by inserting the character sequence "\A", e.g.:

content: "this is the first line\Aand this is the second one.";

Setting the CSS property white-space to pre, pre-wrap or pre-line is required for the character sequence "\A" to be recognized as a forced line break, e.g.:

@page {
  @top-center {
    white-space: pre;
    content: "this is the first line\Aand this is the second one"
  }
}

For page margin boxes in HTML documents this is not necessary, as it is done in the HTML default stylesheet.