Thank you for writing.
One way to accomplish your task is to use the Visual Style Builder
tool and modify the theme. With this approach, however, you will change the disabled back color for all the controls which are using your theme. As I understand you would like to do this on a control basis. In this case, you can refer to the SetThemeValueOverride API: https://docs.telerik.com/devtools/winforms/telerik-presentation-framework/override-theme-settings-at-run-time
For a drop-down button, you can change its disabled back color this way:
I hope this helps. Let me know if you have other questions.