Thank you for contacting us. Visual Style Builder
should support RadTrackBar. Unfortunately, there is a problem with previewing the changes. We know that this leads to difficulties in designing new themes and we apologize for the inconvenience. Out team will do our best to provide solution of this issue in future releases. I have updated your Telerik points. If you need further assistance, do not hesitate to contact us back.
All the best,
the Telerik team