it's clearly that every generation in pokemon have one theme ,
what name you will give to the five generations that we already have?

mine :
Gen I - The Original Generation (because it's the first generation and the original)
Gen II - The Animals Generation (because most of the generation is animals-like)
Gen III - The Nature Generation (because the legendary and some of the pokemon are Nature-related and Weather-related)
Gen IV - The Myth Generation (because of the legenadry pokemons , most of them is based on myths , and the god theme of Arceus , Giratina & more)
Gen V - The Modern Generation - (most of the generation is seemed to have modern theme , and even one of the the legendary pokemon is actually some kind of robot aka Genesect)

what your themes?

Gen 2 is based on Tradition and Gen 5 is based on popular/ cool things (i'm not sure about gen 5 tho but i think they said it in an interview after they discussed Gen 6's beauty theme)

Gen 6 is definitly style. They even confirmed it at E3

if Gen VI is a beauty theme gen , how you can explain Banacale?

That's what they said, and it is obviously the theme, being able to change clothes, beautiful sceneries etc. And not every Pokemon needs to follow this theme, they obviously want to make some badass or ugly Pokemon. It is just the general theme