We rescued a boxer yesterday from a backyard breeder as well..
We breed and show boxers and have rescued many over the years. They are a very muscular lean dog to begin with, but 35 lbs sounds about ten under for a small female, if you have a pic can you post it. You should see very refined musculature and ribs/hips should be defined but not prominent. If you have a tape you can take her height at here shoulders as well and that will give you a good estimate of what she should weight.
As far as dog food, a lot of good comments, i shudder everytime i see someone throwing away $30-50 on corn meal, ie iams, science diet, etc,
As mentioned for fattening up hard boiled eggs are perfect, great balance of protein and fat for them. Their coat will also look great.
We have fed diamond, nutromax, and now feed exclusively canidae. They are all decent food, but with the HUGE increase in the cost of nutromax we now feed only canidae its a little more expensive but GREAT food. With five big dogs in the house, the decrease in yard cleanup duty is well worth it.
You will learn to love the joker of the dog world, boxers are great!