Food struggles are one way that this age group can control their surroundings. Try to avoid a power struggle with your preschooler by offering healthy choices and allowing your child to eat what they want from the choices offered. A child will not starve themselves, they will eat when hungry.
For toddlers who will not expand their eating choices, it's likely not much of an issue. Continue to put new and repeat foods in front of them - some studies have shown it takes 10-20 times for a toddler to accept a food. Stress the healthy foods that they already like and supplement it with a multi-vitamin.
Well, she definitely eats something. She will not let herself starve, so keep offering new foods and eventually she will try them. Make it a rule that she needs to try a bit of everything on her plate every meal.