Hello, Javad,    

Following the provided information, I have prepared a sample project with a RadDock in RTL mode and Windows7 Theme applied. I was able to observe the undesired behavior.

I have logged it in our feedback portal by making this thread public. 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 use the custom theme that is attached in the Windows7RTL.zip file. The following help article demonstrates how to apply a custom theme to your application: https://docs.telerik.com/devtools/winforms/styling-and-appearance/using-custom-themes 
I hope this information helps. If you need any further assistance please don't hesitate to contact me. 

Regards,
Dess | Tech Support Engineer, Sr.
Progress Telerik