Как определить, использует ли пользователь стили?

Вариант 1:

Source: https://www.swissdelphicenter.ch/en/tipsindex.php

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;

Вариант 2:

Author: Alex

Source: Vingrad.ru https://forum.vingrad.ru

uses
Themes;
...
if ThemeServices.ThemesEnabled then // Тема использется