tiny followup to #1103

It works around the current PDF rendering bugs like #1261

For those who can't install Google Chrome (e.g. work network policies) but can use Chromium, Chromium does work for creating the pdf. I updated the readme accordingly. Thanks!

Theming/sky.css has dark gray text on light blue background; not white text

Now the transition isn’t any more „per slide“ but „per movement of a
slide“. So the slide can disappear in another way than it appeared.

To honor the DRY principle repeating patterns have been factored out.
In addition there was an anchor missing in README.md.

Some code blocks weren't indented, so they ended up messing up the numbering. Indenting them fixed it.

renamed markdown attributes for clarity and consistency: data-vertical -> data-separator-vertical, data-notes -> data-separator-notes