I've been poking at my rom com idea. It's difficult to start it in earnest. Because I'm lazy.
Anyway, I've been trying to outline it and such, but I quickly get lost in the second act because I haven't done any research yet (seriously. Who wants to do research after college? No one. Wish I had an assistant already) and without a second act I have no clue what will happen in the third act. The third act of the rom com is usually the point where the two meant to be lovers get together, but not having plotted a second act I'm not sure which candidate it will be, if any.
And I haven't solidified the theme yet. The third act is going to play the theme out to its fullest. Since I don't know the theme, who my protag is going to end up with. Or I could play the theme very realistic and not have her end up with anyone. (I'd love to say I'm writing the next (500) Days of Summer here, but I'm not. Hello, commercialism, goodbye, realistic endings.) My favorite possible theme was just because one guy is a sleaze doesn't mean they all are.
But really, I think it makes for a more enjoyable writing process if the theme is a question, not a statement. It's more fun to have your character grapple with questions like are all guys sleazebags or just the ones she's dated so far? Do all men have dysfuntional relationships or is it really her? Sometimes it's more fun to explore a question than pontificate a theme. (And just so we're clear, those are my character's questions. Definitely not... mine) Then your characters are more than just pawns in your story's soapbox. They are the story.