We are not aware of such a problem. The CssClass setting of the Telerik AJAX controls work and support both RenderMode options - ligthweight and classic.
If the CSS you have is written for the Classic rendering, it will most likely not work since the lightweight rendering is not compatible with the classic one.
Can you please isolate the issue in a simple runnable project and send it for examination on our end along with a couple of screenshots what is wrong and how it should look?
Thank you for your assistance in advance!
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