All RadControls from the Telerik UI for WinForms suite get the ControlDefault theme applied to them by default if no other theme is set. As of R1 2021, it is possible to define which theme is your default theme (e.g. ControlDefault) for the controls even at design time. This can be defined in the App.config file.
As of R2 2021 in the Options dialog, there is a setting that controls whether the message for changing the default theme will be shown.
If "Suggest default theme" is set to true, the following message will be prompted to the user when you drag a Telerik RadControl from the toolbox and drop it onto the form:
If you accept the changes and restart the designer, the Fluent theme will be used as default theme for the design time experience.