Thank you for writing.
I tested your case and I can confirm that this an issue with the Windows8 theme. I have added this case to our Public Issue Tracking System. You can track its status here: http://www.telerik.com/support/pits.aspx#/public/winforms/15158
Currently, you can use the attached theme where this issue is resolved, or you can wait our next release where this issue and the issue that you already reported in this forum thread
are resolved. We are plan to bring the new release to life by the end of the week.
I have added 500 Telerik Points
to your account for this report.
I hope this helps.