Windows 10 dark mode

dark-mode.c

#include <windows.h>
#include <uxtheme.h>
#include <dwmapi.h>

#define DWMWA_USE_IMMERSIVE_DARK_MODE 19

extern IMAGE_DOS_HEADER __ImageBase;
#define HINST_THISCOMPONENT ((HINSTANCE)&__ImageBase)

struct window {
    HWND window;
};

static void handle_nccreate(HWND window, CREATESTRUCTW *cs)
{
    struct window *data = cs->lpCreateParams;
    SetWindowLongPtrW(window, GWLP_USERDATA, (LONG_PTR)data);

    SetWindowTheme(window, L"DarkMode_Explorer", NULL);
    DwmSetWindowAttribute(window, DWMWA_USE_IMMERSIVE_DARK_MODE,
                          &(BOOL) { TRUE }, sizeof(BOOL));
}

static LRESULT CALLBACK window_proc(HWND window, UINT msg, WPARAM wparam,
                                    LPARAM lparam)
{
    struct window *data = (void *)GetWindowLongPtrW(window, GWLP_USERDATA);
    if (!data) {
        if (msg == WM_NCCREATE)
            handle_nccreate(window, (CREATESTRUCTW *)lparam);
        return DefWindowProcW(window, msg, wparam, lparam);
    }

    switch (msg) {
    case WM_CLOSE:
        DestroyWindow(window);
        return 0;
    case WM_DESTROY:
        PostQuitMessage(0);
        return 0;
    }

    return DefWindowProcW(window, msg, wparam, lparam);
}

int main()
{
    ATOM cls = RegisterClassExW(&(WNDCLASSEXW) {
        .cbSize = sizeof(WNDCLASSEXW),
        .lpfnWndProc = window_proc,
        .hInstance = HINST_THISCOMPONENT,
        .hCursor = LoadCursor(NULL, IDC_ARROW),
        .hbrBackground = (HBRUSH)(COLOR_WINDOW + 1),
        .lpszClassName = L"dark-mode-test",
    });

    struct window *data = calloc(1, sizeof(struct window));
    data->window = CreateWindowExW(WS_EX_APPWINDOW, (LPWSTR)MAKEINTATOM(cls),
        L"Dark Mode Test", WS_OVERLAPPEDWINDOW | WS_SIZEBOX,
        CW_USEDEFAULT, CW_USEDEFAULT, 200, 200, NULL, NULL,
        HINST_THISCOMPONENT, data);

    ShowWindow(data->window, SW_SHOWDEFAULT);
    UpdateWindow(data->window);

    MSG message;
    while (GetMessageW(&message, NULL, 0, 0)) {
        TranslateMessage(&message);
        DispatchMessageW(&message);
    }

    free(data);
    UnregisterClassW((LPWSTR)MAKEINTATOM(cls), HINST_THISCOMPONENT);
    return message.wParam;
}

Above 20H1 uses 20 not 19

You have to make a build check

Why doesnt your code work on my end?