I think so far there hasn't quite been enough published to let people distinguish what is a background vs. a theme vs. a scheme.
I'd say I'm in the middle on the class vs. theme spectrum- I'd like to see all the 3rd ed. base classes from the PHB (which to me seem to be the 'core'/'iconic' options people are looking for) as classes, and probably 90% of what is left from 3/3.5e expansions and 4e turned into themes or something similar (not an expert at 4e mechanics for some of the classes, so I'm not sure how plausible this is). Here is how I would like to see it breaking down:
Background: What you did before becoming an adventurer/gaining a class level. If you want a Rogue who grew up in the forest (but isn't specialized in forest roguery) or a paladin who was raised in a monastery as a temple orphan, it goes here. Provides a few skill bonuses, maybe a minor ability.
Class: The core concept and mechanical features of your class- hit/save/damage progression, spell progression, etc. Defines very broadly how you accomplish things- spells, skills, martial techniques, a mix of the above, etc.
Theme: Picks a primarily mechanical specialty within the scope of your class. Stealth focused Rogue (vs. melee damage focused rogue or ranged rogue), generalist wizard (vs. specialist wizard), defender vs. attacker Fighter, ranged Ranger vs. dual wield Ranger, etc. This is primarily mechanical with little RP/fluff/background component.
Scheme: A specialization/focus of your class that has both mechanical and RP-components. This, to me, is where things like Urban Ranger or Cavalier Paladin or Wilderness Rogue would pop up. The Thief scheme that we've seen gives some skills points (specialization, mechanical), a feature that makes him better at one of his abilities (ditto), and an additional RP-focused ability (thieves cant). It also suggests some RP/background options. Urban Ranger/Desert Ranger/Elemental Demiplane of Ranch Dressing Ranger could supply some environment-specific bonuses, Cavalier gives bonuses when mounted or special mount abilities, etc.
So you want a Ranger who was raised by monks, fights with two weapons, and then specialized in urban environments. You pick Ranger, the Temple Orphan Background, the Dual Wield theme, and the Urban Ranger scheme. A rogue who grew up as a farmer, fights like a swashbuckler, and is best at non-urban rogueing? Commoner background, Blade theme, bandit/wilderness rogue scheme.
This is nice because it doesn't conflate the environment of your upbringing with the type of class that you play- a Fighter raised in the desert who becomes a Defender and worked as a mercenary is different from a Fighter raised in the desert who becomes a Slayer and has the Sandstorm Dervish scheme. Both guys know basic things about life in the desert that you would know from growing up there, but only one of them has abilities that are tied to that environment (n.b., I just made up Sandstorm Dervish, and I don't know if Schemes will get extended to be that detailed. He fights with scimitars made of magical sand, though).
