There's no one theme in A Room with a View. But one of the most important themes is the difficulty of reconciling love with one's position in high society. Lucy Honeychurch is very much in love with George Emerson; but according to the standards of her class, love doesn't really enter into the equation when it comes to finding a suitable marriage partner. That's why Lucy ends up within an ace of getting married to Cecil Vyse, who would've been a completely unsuitable husband for her.
It's notable, too, that Lucy falls in love with George in another country, far away from the stifling constraints of Edwardian England. This could be seen as illustrating another important theme in the story: the importance of environment in bringing out one's deepest emotions.
It's unlikely in the extreme that these two young lovebirds would've answered the dictates of their hearts quite so affirmatively had they remained at home. It is only by traveling to another country with a completely different culture and way of life that they've been able to get in touch with their deepest emotions. The emotions were always there, to be sure, but they needed the right environment to bring them to the surface.