Slim down? Meaning they are fat? You can’t get rid of fat in a spot, you have to burn calories and wait for the fat to go away, and your body will hang on to fat in certain paces depending on your genes. Working the muscle has nothing to do with the fat in that spot, except that building muscle helps you burn calories more efficiently. It also can make your legs more shapely, but even if you have strong, shapely, muscles, if the fat is on top of it, your still fat in those spots. Bulking up a muscle will make your thighs bigger, but if you are a girl it takes a lot of work to bulk up, so I would not be too concerned.
So, do aerobic exercise and eat well, lots of veggies and enough protein, count calories, and your thighs should look better. You can walk or job in your neighborhood. Or dance at your house or go out dancing to burn calories and your legs will tone up.