Use the ASP.NET CSS Adapters or Try using both Skins and CSS.
Use a skin to hook CSS selectors (like ID and Class) into your ASP.NET controls, then use a Cascading Style Sheet to set your display properties.
Skins depend on the Style attributes just like Style Sheets, It's not really a case of Skins Vs CSS because without Styles we couldn't have Skins.
A Skin based approach contributes to code bloat since most ASP.NET generated style attributes are rendered into CSS inline styles. A StyleSheet based approach contributes to leaner markup, makes better use of caching, contributes to a faster loading page, can be reused across sites (independent of the programming language or platform), and can easily be maintained by web designers.