I see Ophelia as a very obedient, quiet person, waiting until after she loses her father and her belief in Hamlet's love to show her true self, although considered crazy. The problem I'm having is being able to put this all into one general theme...
To an extent, I agree with your assessment of Ophelia. She is the dutiful daughter and subject to the King. However, I don't see insanity as her true self, but rather a reaction to Hamlet's rejection and her father's murder at the hands of the man she loves.
In Shakespeare's Hamlet, the women of the story are of central importance to Hamlet as he struggles to avenge his father's murder. Ophelia is a victim, as she is used by three men who have no deep concern for her well-being. Hamlet's betrayal is perhaps the worst because he allegedly loves her, whereas her father is a fool. I would expect more from Hamlet's character.
Perhaps the theme that fits best is their mutual failure to look beyond the surface and to trust what they find within the other person.
I find that Hamlet and Ophelia fail to clearly see each other. Hamlet only sees Ophelia's connection to her father, and her duty to her King, and discounts who she is inside and the love she has for him. And it would seem she truly cares for him...why else would his rejection affect her so strongly?
Part of Ophelia's failure to truly see Hamlet may be understandable. Consider who her father is, and what is expected of her socially: to be a dutiful daughter and subject. She is also the product of a father who is full of himself, and a brother who hypocritically expects specific behavior of her which he does not seem to intend to exercise himself. However, instead of considering Hamlet's devastation over his father's sudden death, and his mother's remarriage, Ophelia accepts what she sees at face value and listens to Polonius and Claudius, rather than trying to look beneath the surface of Hamlet's outward behavior.
While Ophelia thinks Hamlet is insane, he never communicates with her what is happening regarding his father's death. Had there been more trust between them, and clear communication, perhaps Ophelia would have been better able to deal with her father's death, as Juliet does when Romeo kills her cousin.
Losing sight of each other and failing to exercise trust based on their past together places a chasm between them which they are never able to bridge.
Perhaps the theme you might investigate is the sublimation of self and the consequences of the sacrifice that comes along with that. Ophelia loved Hamlet, but when both her father and Laertes told her she should stay away from him she obediently did just that. She sublimated her womanly thoughts and emotions, and she lost a bit of herself. When her father died, she had no real relationship left. Her brother was preoccupied with revenge. Hamlet had been rejected, but he too was preoccupied. When she sublimated her true thoughts and feelings, Ophelia lost pieces of herself that were impossible to retrieve.
You might focus on a theme such as family honor that causes the relationship to fail. Hamlet feels that his filial duty is to avenge his father's death. To that end, he decides that he must put on an "antic disposition" and go about verifying Claudius' guilt. This mission puts him at odds with Ophelia, who is clearly obedient to her father, who is very close to the king. Ophelia also abides by the notion of family honor in her loyalty to her father over Hamlet. Polonius seems to convince Ophelia that Hamlet's love to her is not real, that his words are only "springes to catch woodcocks." Polonius fears that Ophelia may ruin the honor of her family by having sexual relations with Hamlet, so he forbids her to see him. Ophelia obeys her father, and breaks up with Hamlet. So, when we see the two of them together in Act 3, scene 1, we see them at the end of their relationship, not at the beginning.
You have an interesting take on Ophelia's character and the question I would ask you and I think you need to answer to create you "theme" is: What do you mean when you say that she "shows her true self." What is her "true" self?
I don't think she is crazy BEFORE the death of father and Hamlet's treatment of her in Act 3. I think you would have a hard time finding textual evidence to support such a claim. I think her craziness comes from the unbearable situation she has endured over the couple of days prior to the breakdown we see in Act 4. I would be a little crazy too if the boy I thought loved me and whom I loved suddenly started acting crazy and then yelled at me to get to a nunnery then spoke rudely and suggestively to me at the play then went and killed my dad! What is she supposed to think! I don't think it is true nature to be crazy; I think the circumstances that driven her mad.
To get to your dilemma about a "theme statement" you need to think about WHY she is so profoundly affected by the events. Think about what you already suggested -- that she is obedient and quiet and then add loyal and I think you are on to something. Someone has those qualities by nature and feels strongly about the people in her life will suffer greatly when people aren't there for her or who emotionally betray her. After all, her father berates her relationship and uses her to spy on Hamlet; and Hamlet says he never loved her.