replace :before pseudo elements with :marker selector

allows to have different marker based on the level of nesting (as in other themes : disc, square, circle)

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.