The css rendered for the telerik controls comes from the Telerik.Web.UI.dll for versions prior to the current one. In the latest versions they come from Telerik.Web.UI.Skins.dll.
However, if you want to see the css files in full, you'll find file-based copies under your installation directory in the /Skins folder.
If you search this site for "Create Custom Skin" you'll find lots of help on creating a custom skin for which ever controls you're using.