Since there are no header sections for user controls in asp.net, user controls have no way of knowing about stylesheet files. So css classes in the user controls are not rec
If you are creating composite UserControl, then you can set the CSSClass property on the child controls..
If not, then you need to expose properties that are either of the Style type, or (as I often do) string properties that apply CSS at the render type (i.e. take them properties and add a style attribute to the HTML tags when rendering).