Same as any other CSS style you can modify the built-in RadListView styles either in the <head> tag of your page or inside .css files included in your project and registered in the page's <head>. The only thing you should take care of is to use high enough specificity of your CSS selectors when trying to overwrite css styles.
I believe that this blog post will give you a good view on things:
How To Override Styles in a RadControl for ASP.NET AJAX' Embedded Skin
the Telerik team