My first inclination is to say the theme is the transformative power of love (since this romance is what propels Mia to believe in herself and take career chances). But fools who dream also makes sense.
Theme seems to be where the movie is weakest, and for the people who don’t like La La Land, that seems to be a point around which a fair amount of the criticism revolves — that it ultimately lacks depth.
The film is lovely and fun (and there’s nothing wrong with just being that — escapism is valid), but the ending is what makes it interesting and worth discussing, IMO. If it had ended with Mia and Sebastian staying together AND being successful, I think La La Land would be fairly forgettable (which is to say, enjoyable but not a record-breaking Oscar contender). The ending also complicates the theme. Love is transformative, but perhaps the transformation wasn’t wholly positive; Sebastian and Mia seem sadder rather than happier at the end. And as you point out, they achieved what they wanted, but perhaps only to find it wasn’t what they really needed.