I was just coming here to make a thread about this.
For me, the white background is always transparent on the BIONICLE subforum when viewing on my Windows Phone. (Strangely, it's fine on my iPad, which presumably uses the same mobile site. Something to do with the screen size, perhaps?)
I've had this problem since the message boards were messed around with for the TTV Civil War.
Naturally, I blame Kahi.