For the most part, if theres no health reason as to why shes underweight, your best bet is to just feed her normally. Unlimited hay in a nice pile on the floor for her so it's easy to reach, a good variety of nutritious veggies, id hold off on watery ones such as lettuce, cucumber etc until you're seeing she's eating lots of hay, root veg is good for weight gain, carrot and beetroot (raw) especially so giving those twice a week or so would be good.. And 1/8 of a cup of pellets. Remember you're wanting her to put on healthy weight, rather than fat so high calorie foods arent necessarily going to help, plus putting on too much weight too quickly can make her poorly, there is something called re feeding syndrome which has something to do with the glucose in blood, so nice gentle weight gain will help keep everything in sync. Alfalfa hay will help if you can get hold of it, it is higher in protein than grass hay so will help her build muscle mass back up, but contuinue offering grass hay alongside this. If you can't find the hay itself, alfalfa based pellets are ok to feed until they're about 6 months old.
Her rough coat could well be due to malnutrition so some feedings of critical care will also help, especially the fine grind as you can mix it thicker and get more nutrition into them in smaller amounts. You can mix in some raw rolled oats or wheat bran but not too much, just a little pinch will do.
With any luck, with proper care she should be healthy and happy in no time. Probiotics may also help, fibreplex, proC, biolapis are good ones if you're in the UK.
she’s such a cutie