How do I add a CSS class to a textbox in C?

In a C application, particularly when dealing with a graphical user interface (GUI) framework or library, you can apply styles to UI elements such as textboxes by using the features provided by that specific framework or library. Here's an example using WinAPI, which is a common API for GUI applications in the Windows operating system.

Let's say you're working with a WinAPI application using C. To add a CSS-like class to a textbox, you would typically use the `CreateWindow` function to create the textbox and then modify its style using the `SetWindowLongPtr` function.

Here's a basic example:

```c
#include <Windows.h>

LRESULT CALLBACK WindowProc(HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam);

int main() {
    // Create a window
    HWND hwnd = CreateWindowEx(
        0,                              // Optional window styles
        L"EDIT",                        // Class name
        L"Sample TextBox",              // Window text
        WS_VISIBLE | WS_OVERLAPPEDWINDOW, // Window style

        // Size and position
        CW_USEDEFAULT, CW_USEDEFAULT, 400, 200,

        NULL,       // Parent window
        NULL,       // Menu
        NULL,       // Instance handle
        NULL        // Additional application data
    );

    // Check if the window was created successfully
    if (hwnd == NULL) {
        return 1;
    }

    // Set a CSS-like class (modify the style)
    SetWindowLongPtr(hwnd, GWL_STYLE, GetWindowLongPtr(hwnd, GWL_STYLE) | WS_BORDER);

    // Show the window
    ShowWindow(hwnd, SW_SHOWNORMAL);

    // Run the message loop
    MSG msg;
    while (GetMessage(&msg, NULL, 0, 0)) {
        TranslateMessage(&msg);
        DispatchMessage(&msg);
    }

    return 0;
}

LRESULT CALLBACK WindowProc(HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam) {
    switch (uMsg) {
        case WM_DESTROY:
            PostQuitMessage(0);
            break;
        default:
            return DefWindowProc(hwnd, uMsg, wParam, lParam);
    }
    return 0;
}
```

In this example:

- `CreateWindowEx` is used to create a basic window with a textbox.
- `SetWindowLongPtr` is used to modify the window style. In this case, `WS_BORDER` is added to give it a border, somewhat similar to applying a CSS class.
- The message loop (`GetMessage`, `TranslateMessage`, and `DispatchMessage`) runs to keep the window open and responsive.

Please note that this is a basic example using WinAPI for Windows desktop applications. If you are working with a different GUI framework or library, the approach will likely be different. Additionally, modern applications often use more advanced technologies like WPF or GTK, which provide more sophisticated ways of styling and theming.