New theme using color palette from [Dracula](https://draculatheme.com/)
dark editor theme available for quite a number of different
applications.

When embedding reveal in a website, this will help avoid global variable conflicts with other frameworks that use global styles via variables.

Fixed broken URL

* Print background in pdf

Pull request to integrate issue https://github.com/hakimel/reveal.js/issues/1686. Adding the workaround to the theme template to make it work for every theme.

* apply suggestion of @hakimel

Closes #1951.

"proceding" → "proceeding"

This reverts commit b7e0d9b1.

Defines the highlight colour when selecting slide content using Mozilla
Firefox to match other browsers, instead of being the default colour.

This is similar to what is done in the "white" theme.