To reproduce: 1. Add a RadCollapsiblePanel and change the back color or border of the container via the Edit UI elements dialog. 2. Save the changes and run the project. You will notice that the style is reset and the changes are not preserved. Workaround: Customize the style at runtime. The following help article gives you information about the internal elements structure of the RadCollapsiblePanel: https://docs.telerik.com/devtools/winforms/panels-and-labels/collapsiblepanel/structure
Serializing the properties of ControlsContainer is not a good approach, because it adds too much code to the designer.cs (please refer to the attached screenshot) This is why we decided to removed the option to interact design time with the ControlsContainer. Instead you can customize the styles at run time. The following help article gives information about the internal elements structure of the RadCollapsiblePanel: https://docs.telerik.com/devtools/winforms/panels-and-labels/collapsiblepanel/structure