Like any structure, the beats for Save the Cat are guidelines, not rules. If breaking them would serve you or the story better than following them, then absolutely break them! For Save the Cat specifically, the "Opening Image" is just a snapshot of how things are at the start, which can be effectively contrasted to how they are in the closing image for deeper meaning. The wider "setup" beat that precedes the inciting incident can (and often does) take multiple scenes or chapters.

Definitely plan for what the story needs! If, later, you realize that the setup was too long and slowed the story's pace, you can always tighten things up in revision.

For working the theme in, that can be a challenge! Do you already know what important change your MC will experience over the course of the book? How will they and their world change by the end? If you know that, it's much easier to find a way to allude to it or have a side character say a line that foreshadows/saucily winks at the growth your protagonist will experience!

For example, if your MC is rigidly conformist and believes that the only way to do good is to do exactly as they're told—and as the story goes on, they realize that blind obedience isn't always the way and that sometimes they have to think for themselves and make hard decisions, the opening image could have a scene where the MC displays those qualities and a side character jokingly shoves them and tells them to loosen up or something, and they respond with anger...which could be an effective contrast to a book's final scene, where the character has, in fact, "loosened up" and changed drastically after learning some hard lessons. (That's a bad example, but it's one way, off the top of my head, that an opening image might be handled.)