avoid centering of text when the slides are zoomed out

When pressing [esc], the text on the tiny slides appears centered, because of a rule that contains

.reveal .slides {
  text-align: center;
}

which is inherited from Reveal.js default theme. In turn, the slide thumbnails look completely different from what's actually displayed. This is the minimal css change that fixes it for me. Not necessarily ideal, but I probably lack the high-level view to judge. :]

assigned to @laurent.heirendt

mentioned in commit f772cd1f

merged