Style is the key to the seemingly odd assortment of pieces that American Ballet Theatre chose to open its Wolf Trap season on Monday and then repeated last night -- except for a different pas de deux and major changes in casting. Ballet technique and a romantic relationship are at the core of each work and there is an absence of detailed plot and specific characterization. The danger of such a bill is that everything may look too much alike, for even the most lyrical of these ballets, Fokine's "Les Sylphides," can be demystified, as George Balanchine once proved in a wickedly clever production.

Nuance and atmosphere were more apparent in the ensembles of "Theme and Variations" and "Raymonda" yesterday, although "Theme" is far from the gem of the ABT repertory that it once was. This production, with its Santo Loquasto costumes and Thomas Skelton lighting, gives the impression of night much of the time while Tchaikovsky's music and Balanchine's steps have the clarity and brightness of sunlight. The corps, though more harmonious than on the first night, seems to have settled for an awkward compromise between the angular arms of Balanchine's "Agon and after" manner and the more rounded carriage of the classical school. Moreover, mere speed is often foremost in the group work. Yet there are pleasures. Martine van Hamel, in the ballerina/princess role, showed that it is more critical to be rhythmically alert than quick in Balanchine. Patrick Bissell, her suitor, was closer than in Monday's "Raymonda" to the expansive ease that was once second nature for him.

The star performance was Cynthia Gregory's rendition of the ballerina/princess in "Raymonda." Gregory has now lost all traces of girlishness, and this is most becoming. Her technical mastery courts rather than challenges the audience. Not least, Gregory knows just how much folk dance flavor to add to Petipa's classicism. The right combination gives "Raymonda" its unique style.

"Les Sylphides" suffered from changes in casting. Amanda McKerrow hasn't yet made the austerity of the first sylph her own. Intelligence and musicality showed throughout her performance, as did the new way she plays with hands, arms and shoulders, but temperamentally she's right for the more flirtatious sylph she danced on opening night. Her two companions, Leslie Browne and Lisa Rinehart, were adequate, and Peter Fonseca repeated his smooth but much too remote portrayal of the poet/dancer.

"Don Quixote" was the pas de deux, but the couple who danced it didn't click as a partnership. Cheryl Yeager showed her spunk only in her solo variation. Danilo Radojevic had wallop but no rebound. ABT presents a revised "Giselle" tonight and the season continues through Saturday, when new principal dancer Alessandra Ferri appears in "La Bayadere."