A previous blog post on background and themes made it pretty clear that backgrounds and themes are just going to be respective packages of skills and feats, though other information drops hinted--or could at least be interpreted--that they might confer other benefits as well.
This post states that themes and backgrounds "also give a character class access to the feel that's traditionally been in the keeping of another class". It then explains that in prior editions if someone wanted to play a spy that they would probably start as a rogue, or at least multiclass into rogue as soon as possible.
I can agree with this statement--though not with the one about a wizard of any edition acting like a spy, especially "to great and wonderful effect"--because rogues get a lot of skill points/trained skills that make them ideal at sneaking, lying, and information gathering. So, yeah, if any class can pick any skills then it opens up a lot of concepts that you might be unable to easily and/or reliably realize. It also clarifies that classes must have a core identity and mechanics that are fundamental to that class.
In other words this sounds like "classes give you class features". I wonder if feat choices will let you pick up class features from another class, like in 4th Edition, or boost logical/thematic/iconic combinations of two or more other classes, like fighter/wizards, rogue/assassins, druid/barbarians, ranger/druid/paladins, etc.
Ultimately what I take away from this short post is still that backgrounds and themes largely exist to speed a player through character creation. What I do not know for sure is the order of operations; you can pair of any background and class, but what about themes? From the sounds of the slayer theme I guess that classes will at least have specific theme options, which I liken to sample character builds from 3rd and 4th Edition, though I could also see them having generic or broad themes that are largely applicable to multiple classes.
What I am curious about is if backgrounds will do more than just dole out skill bonuses. For example, will the spy background provide a network of contacts? Will themes only determine your feats, or will they also provide other benefits like they did in 4th Edition? Does a wizard with the farmer background still have access to Arcana, or whatever a wizard uses to deal with magic stuff? Can a fighter start with the slayer theme, but then move more into the realm of archery if the campaign direction warrants it?