ThemeResolutionService.ApplicationThemeName = "MaterialPink";
            radGridView1.ElementTree.EnableApplicationThemeName = false;
            radGridView1.ThemeName = "FluentDark";

        private void radGridView1_CellEditorInitialized(object sender, GridViewCellEventArgs e)
        {
            RadDropDownListEditor ddlEditor = e.ActiveEditor as  RadDropDownListEditor;
            if (ddlEditor != null)
            {
                RadDropDownListEditorElement el = ddlEditor.EditorElement as RadDropDownListEditorElement;
                el.Popup.ElementTree.EnableApplicationThemeName = false;
                el.Popup.ThemeName = this.radGridView1.ThemeName;
            }
        }