The theme is: Canadian SF Coast to Coast, i.e., regional representation of SF from across Canada--which translates out to a pretty open-ended theme.
I have to say that Edge has done a good job of continually appointing excellent editorial teams. Vernon and Anderson are both respectable names in the field; we have gender balance; reasonable regional balance; poetry and short fiction both well represented; and yet very different tastes, so presumably anything that they can agree on will have to be good enough to transcend particular subgenres. So I'm pretty pleased with the choice, once again.
The rotating editorship does seem, well, a pecularly Canadian approach. I can't think of any other on-going anthology series that switches editors every edition. I think that is one of the things that keeps the series so fresh, and gives it such staying power. You didn't like this year's theme or felt that it was not sufficiently representative of 'X'? No problem! Next time it will have a different theme, different editors, and whatever that perceived gap was, sooner or later, the series will get there. (E.g., not enough stories from Quebec? See TesseractsQ. Nowhere to publish your novella because its too long for an anthology series? No problem, they had one for novellas. I'm telling you, the Tesseracts approach is just brilliant!