The issue is caused by the fact that the drop down button has its own theming in RadDock, which gets applied to the newly added button.

To reproduce:

 protected override void OnLoad(EventArgs e)
        {
            base.OnLoad(e);

            RadDockEvents.TabStripItemCreating += RadDockEvents_TabStripItemCreating;
            AddDock();
            ThemeResolutionService.ApplicationThemeName = "VisualStudio2012Dark";
        }


        void RadDockEvents_TabStripItemCreating(object sender, TabStripItemCreatingEventArgs args)
        {
            RadDropDownButtonElement btn = new RadDropDownButtonElement();
            btn.Margin = new System.Windows.Forms.Padding(80, 5, 5, 5);
            btn.MinSize = new System.Drawing.Size(50, 20);
            btn.Items.Add(new RadMenuItem("asasda"));
            args.TabItem.Children.Add(btn);
        }

Workaround: use the themes in the attachments