to start, phpbb.com has nothing to do with forum2go.nl they just happen to use the phpbb software.
that style is proflat.
there is no way that your style was changed accidentally.
however, forum2go very well may have upgraded their version of phpbb and also done something to switch the style.
it is not a hack.
contact your host forum2go.nl
I am available for custom work on a donation basis. Please send me a PM with your needs.
Premium phpBB 3.2 Styles
OK, so what's the speed of dark?