Fix print to PDF view in browser

[cderv]

https://revealjs.com/pdf-export/

I don't think it is currently working well. Especially with background color and image.

More precisely:

• When using print PDF view, by adding ?print-pdf at the end of the url
• It does not show very well in browser
• But seems to print ok when printing is done in chrome 🤔

This could be fixed with an upgrade to revealjs and our template

[cderv]

It seems it is not better with last reveal and pandoc's template 🤔