Me.IsMdiContainer = True

Hi, Mahdi,

RadForm, similar to the standard MS Form, can be specified as a container for MDI child forms. Thus, the default back color for the form is changed to gray. 

I have reviewed general programming forums to see how this color can be customized. Please refer to the following articles which are quite useful on this topic:

https://docs.microsoft.com/en-us/dotnet/api/system.windows.forms.form.ismdicontainer?view=net-5.0

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