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)

fixes #2867

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

fix fullscreen issues; correct size in Safari, auto-focus presentation when entering fullscreen #3080

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