RevealJS CSS Update

[Andrew Scholer]

A heads up for anyone working on RevealJS. A PR was just merged to more the CSS out of the HTML file into a separate CSS file. That file is built by the same script as the other CSS.

There should not be any visible styling differences related to the switch. It should however ease development/maintenance/experimentation.

There is also now a publisher variable to specify the Reveal theme to use as a starting point:
https://revealjs.com/themes/

See original discussion here:
https://groups.google.com/g/pretext-dev/c/ikaLOw8hta8/m/4pj-45hMBgAJ

Andrew