Abstruse
Legend
So the ranger class is nothing more than "Guy who hangs out in the woods a lot" then?Someone else pointed out that a theme discusses HOW you do something, not WHAT you do. Yeah, any permeation of the ranger is going to put you in a green cloak, because that is what a ranger DOES. The theme describes if he's an archer, a dual-wielder, a slayer, a guardian, a lurker, etc.
A reverse example: what does a "ranger" theme look like? What does a paladin theme look like? Are we willing to make things like detect evil or lay on hands feats?
For the reverse example, here's my idea on a ranger theme off the top of my head (two sets, one called Robin Hood for the traditional archer ranger and one called Drizzt for the two-weapon fighting ranger).
Robin Hood:
Base: Proficiency with longbows, shortbows, and light armor. +1 to hit and +2 damage with bows.
1st Level: Track - You can attempt to locate tracks using a single action rather than taking a full minute for every 5'x5' square.
3rd Level: Point-Blank Shot - When you fire your bow while adjacent to an enemy, you do not have disadvantage on the attack.
5th Level: Precise Shot - If you have advantage with a bow attack, you do an additional 2d8 damage.
Drizzt:
Base: Proficiency with martial melee weapons and light armor. Can make 2 attacks per round (one for each weapon) at a -2 penalty to each attack.
1st Level: Track - You can attempt to locate tracks using a single action rather than taking a full minute for every 5'x5' square.
3rd Level: Two-Weapon Defense - You gain +1 AC when wielding two weapons.
5th Level: Improved Two-Weapon Fighting - You no longer take a penalty when making attacks with both weapons.
Woodsman Background: +3 to Track (only in the wilderness), Survival, and Wilderness Lore.
Urban Hunter Background: +3 to Track (only in cities, villages, towns, or dungeons), Gather Information, and...whatever would let them figure out information on humanoid or underground monsters.
Please note that since we don't have the character generation rules, we don't know precisely what is granted by a theme. The above is my best guess based on the extrapolations done by myself and others (the thread's around here somewhere if you look). We also don't know at what levels your theme would give you new abilities since we've only seen the pregens up to level 3. Also, this is just off the top of my head with no attempt to balance or work anything else out. I was asked for an example so I gave it. I could probably do a better job, but it would be pointless since I don't work for WotC and they're going to release their idea of whatever the Ranger will be as soon as it's ready and I'd rather focus my efforts to making a D&D Next version of Temple of Elemental Evil for my group.