Printing is broken in Firefox and Edge

[andrewlock]

A niche issue I know, but someone just flagged it to me on my blog. It prints fine on Chrome.

If you view the theme in Edge and press ctrl+p, the resulting print preview is mostly white, only shows the header of the blog, and is only 1 page long.

I've checked on both our blogs, so it's definitely a theme issue - we probably just need to add some print media query css