fix dracula li markers

[t-fritsch]

when working on my presentation I noticed that dracula.css was modified because sass file hasn't been altered in #3450 (only the built css file has been edited)

At first this PR aimed only to simply apply the same patch to the sass file to make it match the compiled version, but then I noticed that dracula's way to render <li> marker was by adding :before pseudo elements resulting in always the same bullet point, whatever nesting level it was into (always a disc). In other themes, the li renders differently depending on the nesting level (disc, square, circle).

By replacing :before pseudo elements with ::marker selector, we can have the same style as in other themes :

from left to right : before this PR / PR / original "dark" theme for comparison

NB: The bullets are now bigger than before this PR, but it is consistent with other themes. If one prefers smaller markers, it can be done in custom CSS :

.reveal ul li::marker {
    font-size: 70%; /* or whatever scale fits */
}

[t-fritsch]

Just noticed there is a regression on ol numbering. Will fix before marking ready for review.

[t-fritsch]

fixed ol numbering

did some sass syntax optimization too (remove duplicate .reveal selectors)

[hakimel]

Oops, completely missed that that #3450 only changed the source files.

Thanks for catching that and for making the styles consistent with other themes.

[t-fritsch]

my pleasure !