Howto assign style to media="print" section?

[Ben]

Normally I can assign new/different CSS styles to a webpage by writing a command like:

GM_addStyle(".classname123 { font-size: 16px !important; }");

Ok.

But what if I want to assign a new CSS value to the media="print" section?

In detail I want to assign the following style:

<style media="print">
element(s) {
page-break-before: always; }
</style>


How does that work?

Thank you
Ben