YMapsML allows you to set the appearance of objects displayed on the map. The appearance of displayed objects is changed using styles. Styles let you set certain parameters for displaying geo objects and generate custom HTML representation using templates (see the example of using templates).
To specify which style to apply for displaying an object, set the ymaps:style element in the ymaps:GeoObject element. It should contain a link to a style that was defined in the repr:Style element. The style for a collection of geo objects can be set the same way in the ymaps:GeoObjectCollection element.