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'm baaaaaccckkkk. still doing work on donation basis. PM your needs.
Premium phpBB 3.2 Styles
If a tree falls in the forest and nobody is there, does it make a sound?