default theme: does not work with browser history

[serhei]

A minor issue worth noting: paging between slides adds an entry to the browser history, but pressing the browser's 'Back' button does not change to the previously seen slide.

This isn't ideal for archiving a set of slides on a website (the longer you page through an archived set of slides, the more times you have to click 'Back' to return to whatever HTML page sent you to the slides; clicking 'Back' with no visible effect is confusing etc).

Not sure if this affects default theme only or the other ones as well.