It seems a little difficult to think of patricide as the real theme since Synge's play is a humorous satire meant to evoke laughter. While patricide is a common theme in tragedy, (e.g., Oedipus Rex), it seems incongruous for Synge to introduce it as an important theme in a satirical comedy. Patricide would more plausibly be analyzed as (1) the off-stage dramatic inciting incident and (2) a motif related to the themes of impulsivity, imagination, and group mentality. Love and marriage may be considered a theme, however, they seem to be secondary ones that support the significant theme of impulsivity since impulsivity is the thematic motif that comprises the off-stage inciting action.
The theme of impulsivity has weight because it is Christy's impulsivity that caused him to strike his father; it is the villagers' impulsivity that generates each of their changeable reactions to Christy; it is Pegeen's impulsivity that causes her to accept Christy's proposal of marriage; and it is the villagers' impulsivity that crowns Christy the Playboy of the Western World.
WIDOW QUIN — [with the shade of a smile.] —
They're cheering a young lad, the champion Playboy of the Western World. [More cheering.]
The most powerful themes are those of imagination and group mentality. It is imagination that forces Christy to cry out that he had only killed his father as a defence against all manner of other imagined offenses impulsively suggested by the villagers' group mentality.
[twisting round on her with a sharp cry of horror.] — Don't strike me. I killed my poor father, Tuesday was a week, for doing the like of that.
Further, it is group mentality that imagines Christy as the Playboy of the Western World. While other themes include rebellion and social convention, imagination and group mentality, closely associated to the theme of myth making, by far carry the greatest weight. Bear in mind that group mentality is defined as expressions of ideas and feelings of the moment, as exemplified by the reaction to Kennedy's speech at the Berlin Wall, while social convention is represented by traditional behavior.