fix dracula li markers

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)

fix code blocks font when printing pdf

fixes #2867

Fix dracula's theme list-style on sub-items

Update demo.html

switch themes keep current slide

Refactored var to let or const, used strict equality operator

fix typos in variable names

fix scss watch tasks broken on syntax error

speed up livereload

connect.reload needs a stream of files to fire, but this stream is irrelevant here and slows refresh time a lot (from ~2ms to 2000ms here)