function _IsThemeActive: Boolean;
// Returns True if the user uses XP style
const
themelib = 'uxtheme.dll';
type
TIsThemeActive = function: BOOL; stdcall;
var
IsThemeActive: TIsThemeActive;
huxtheme: HINST;
begin
Result := False;
// Check if XP or later Version
if (Win32Platform = VER_PLATFORM_WIN32_NT) and
(((Win32MajorVersion = 5) and (Win32MinorVersion >= 1)) or
(Win32MajorVersion > 5)) then
begin
huxtheme := LoadLibrary(themelib);
if huxtheme <> 0 then
begin
try
IsThemeActive := GetProcAddress(huxtheme, 'IsThemeActive');
Result := IsThemeActive;
finally
if huxtheme > 0 then
FreeLibrary(huxtheme);
end;
end;
end;
end;

// Example Call:

procedure TForm1.Button1Click(Sender: TObject);
begin
if _IsThemeActive then
ShowMessage('Windows Themes are active.');
end;