This one should be easy enough.
You post something that is the opposite of what the next poster puts up.
They then have to find a link between between what the previous poster has said and the theme at the time.
Anyone can choose a new theme but the theme can only change each time the previous poster runs out of differences between the theme & the original post.
They don't have to be differences or similarities they can just be something related or not.
Whoever chooses the new theme must also give a reason for choosing that particular theme. At which point the previous poster can either challenge the next person to find another link between the previous posts or choose another theme.
And then it just carries on like that.
I'll start
Theme: Boredom
I need to get a life.