It's probably not what you want to hear, but I'm inclined to say don't worry too much about it. It depends a little, of course, on exactly how thin she is.
If you're seeing the outline of the last two or three ribs when she is standing normally, then she is actually a perfect weight. It is quite normal on young boxers to see more than that though, especially if your girl was the "type" that grew upward quickly as a puppy. Many boxer pups put all their energy, it seems, into growing taller quickly and they end up very ribby as a result. No amount of feeding will change it though, and it's only after they finish growing upward (at around 18 months) that they slowly start to fill out - and that can easily take until they're three years old or more.
So unless you're seeing vertebrae and hip bones, I would just give her the best quality food you can (personally, I'd stick with raw) and let time and nature take it's course. It is so very much better for a dog to be lean than fat, especially if they're active.
One thing you could consider though, is spaying her. Aside from a wide range of health benefits, that does also tend to slow the metabolism a bit and you may find she gains weight more easily (or more accurately, requires less food to maintain a correct weight).