Judging by the provided information, I conclude that you are using non-embedded custom skins. Since the RadControls for ASP.NET AJAX do not register custom CSS files automatically, the organisation and physical location of the CSS files and images is entirely up to your preference. Generally, you can have several CSS files in one skin folder, but I don't recommend mixing different skins and their images in one folder.
By the way, you can consider using embedded custom skins
. This will bring the following benefits:
1) there will be no need to take care of the physical location of the CSS files
2) you will be able to combine the registered skins on the page with RadStyleSheetManager, so that the number of HTTP requests is reduced.
There is one drawback though - since the CSS files will not be located inside the ASP.NET theme anymore, they will not be registered automatically by the .NET framework. You will have to register them with RadStyleSheetManager
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