Author: Jonas Bilinkevicius 

How to get notified when the user changes the theme (XP)?

Answer:

1   const
2     WM_THEMECHANGED = $031A;
3   
4   type
5     TForm1 = class(TForm)
6       {...}
7     private
8     public
9       procedure WMTHEMECHANGED(var Msg: TMessage); message WM_THEMECHANGED;
10    end;
11  
12    {...}
13  
14  implementation
15  
16  {...}
17  
18  procedure TForm1.WMTHEMECHANGED(var Msg: TMessage);
19  begin
20    Label1.Caption := 'Theme changed';
21    Msg.Result := 0;
22  end;