Intro.js has a bug causing tooltips appear in wrong places (or out of sceen)
when using 'position: fixed' on target element and not being scrolled to top.
This fixes the bug taking window scroll into account. The only place where
it can be seen is during the highlight animation when not scrolled to top,
which is not serious and not very likely to be seen.
[MAILPOET-1446]
The intention here is to use WP default styles (adding .button class to buttons)
with as little overrides as possible, avoiding to use absolute px sizes.
[MAILPOET-1446]
