I don't think that anything as generic as TWFing or S&B or the like should consume a whole Theme. The mix of weapons you use is something that could reasonably vary based on what you're doing. You don't hunt grouse with a broadsword or stand in a phalanx with a rapier & main gauche or keep a two-handed-sword in-hand as you crawl through 3' high tunnels. If fighters are to have any hint of the flexibility they need to keep up with the challenges of adventuring (and the ever-growing selection of spells casters have to obviate those challenges), they'll at least need the flexibility to change weapons, and tying the huge chunk of character-definition and character ability that a Theme represents to one very generic 'style' blows that.
It also means a weapon-oriented character will face a stark choice between being truely effective with whatever style he favors and being customized, at all.
A 'Style' might make sense as a theme if it carries a lot more with it than just what you hold in your hands in a fight. It should have some out-of-combat implications, for one thing, since the fighter, in particular, is in desperate need of those. It should cover several different weapon mixes for different circumstances appropriate to it, as well. For instance, a "Knight" or "Cavalier" theme might include all of the following: Skill in courtly graces, knowledge of military and political history, bonuses for leading men in combat, knowledge of engineering and siegework, hunting including falconry, horsemanship and mounted combat. In addition to mounted combat with the lance in a joust, and lance and other weapons as part of a unit in open war, the Knight should be adept at fighting afoot in a duel or vs many opponents, using weapon & shield or a greatsword or even a weapon like the Bec de Corbin, and should be well-able to defend himself with just a longsword or quarterstaff when not in full harness for battle. Since it's a fantasy world, having specific abilities of use in facing giants, dragons, and the like would also not be unreasonable.