Bug: the display: "flex" config option does not work correctly in pdf-print mode

[aterenin]

Addingdisplay: "flex" to Reveal.initialize and adding ?print-pdf to the URL does not result in section having computed style display: flex;, because it gets overridden to display: block!important; somewhere else in the CSS file.

This causes slides that expect this style to not be displayed correctly. I'd be happy to try to fix this with a PR, but not sure precisely how Reveal internals are intended to work here, and where this should be overridden.