You can use the DecreaseHandleStyle and the IncreaseHandleStyle properties exposed by the RadSlider control. The buttons actually are RepeatButtons and the custom Styles should target the correct type. Furthermore, you will be allowed to define custom ContentTemplates to those buttons.
For your convenience we prepared a sample project achieving your requirements. Please take a look at it and let us know if you need any further assistance.
Regards,
Pavel R. Pavlov
Telerik
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