As the default theme is an embedded resource, it's not possible to extend it. However, you can simply add the CSS inside the template you'll be using, via a CSS widget. This way your additional CSS will be loaded on every page using this template. Another option is to build a custom theme that's based on basic (you can inspect the CSS that's loading using a tool like FireBug). Please let us know if you have any additional questions on this topic, I'll be glad to advise you further.
All the best,
the Telerik team
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