Thank you for writing.
It seems to be an issue with the theme for the clear button. There is no image applied to the button element. I have logged it in our feedback portal
. You can track its progress, subscribe for status changes and add your comments on the following link - feedback item
I have also updated your Telerik points.
Currently, the possible solution that I can suggest is to set text for the button:
I hope this information helps. Should you have further questions I would be glad to help.