With the new data leak, all available cover legendaries have their own themed background. People are speculating that these will be available during the ‘Road to Kalos’ event. But if every generation gets one day (i.e. day 1:Ho-oh and lugia, day 2: Groudon and Kyogre, day 3: Palkia and Dialga), then there are still 2 days left. So, I think that some background might get recycled.
I incirporated a pocture from u/GoodMornEveGoodNight
by salalander