To reproduce: 
1. Run Demo application >> TreeView >> Drag & Drop.
2. Change the theme to Windows7 and start dragging a node.

The drop hint is missing.

Workaround: apply the theme in the Form.Load event after getting the TreeViewElement.ItemDropHint property coming from the ControlDefault theme.

RadImageShape hint;

private void Form1_Load(object sender, EventArgs e)
{
    hint = this.radTreeView1.TreeViewElement.ItemDropHint;
    this.radTreeView1.ThemeName = "Windows7";
    this.radTreeView1.TreeViewElement.ItemDropHint = hint;
}