SetWindowRgn changes window theme to Classic - c++

Question

When i use the SetWindowRgn function in my windows api code, the created windows style changes from Modern to Classic.
Minumum code to reproduce problem here:
LRESULT CALLBACK WndProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
{
switch(msg)
{
case WM_CLOSE:
DestroyWindow(hwnd);
break;
case WM_DESTROY:
PostQuitMessage(0);
break;
default:
return DefWindowProc(hwnd, msg, wParam, lParam);
}
return 0;
}
int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance,
LPSTR lpCmdLine, int nCmdShow)
{
WNDCLASSEX wc;
HWND hwnd;
wc.cbSize = sizeof(WNDCLASSEX);
wc.style = 0;
wc.lpfnWndProc = WndProc;
wc.cbClsExtra = 0;
wc.cbWndExtra = 0;
wc.hInstance = hInstance;
wc.hIcon = LoadIcon(NULL, IDI_APPLICATION);
wc.hCursor = LoadCursor(NULL, IDC_ARROW);
wc.hbrBackground = (HBRUSH)(COLOR_WINDOW+1);
wc.lpszMenuName = NULL;
wc.lpszClassName = g_szClassName;
wc.hIconSm = LoadIcon(NULL, IDI_APPLICATION);
if(!RegisterClassEx(&wc))
{
MessageBox(NULL, "Window Registration Failed!", "Error!",
MB_ICONEXCLAMATION | MB_OK);
return 0;
}
hwnd = CreateWindowEx(
WS_EX_TOPMOST | WS_EX_CLIENTEDGE,
g_szClassName,
"Capture Screen",
WS_OVERLAPPEDWINDOW,
10, 100, 800, 600,
NULL, NULL, hInstance, NULL);
if(hwnd == NULL)
{
MessageBox(NULL, "Window Creation Failed!", "Error!",
MB_ICONEXCLAMATION | MB_OK);
return 0;
}
ShowWindow(hwnd, SW_NORMAL);
UpdateWindow(hwnd);
HRGN WindowRgn = CreateRectRgn(0,0,800,600);
SetWindowRgn(hwnd,WindowRgn,TRUE); // <===== comment, uncomment this line to observe difference
MSG Msg;
while(GetMessage(&Msg, NULL, 0, 0) > 0)
{
TranslateMessage(&Msg);
DispatchMessage(&Msg);
}
return Msg.wParam;
}
Commenting and uncommenting the line below will demonstrate the problem
SetWindowRgn(hwnd,WindowRgn,TRUE); // <===== comment, uncomment this line to observe difference
Modern (commenting the line) :
Classic (uncommenting the line) :
Any idea what exactly in the function causes the problem here? And any fixes for this?