How do I set a background color for the whole window of a Qt application?

Does anyone know how one would be able to set a background color for the whole window of a Qt application?

So far I am using stylesheets but can only figure out how to assign a background color to a widget such as QGroupBox or QPushButton. Basically, if I want a black background how would I make it seamless without any borders of the original background?

I would simply use a Style Sheet for the whole window.

For instance, if your window is inheriting from QWidget, here is what I'm doing :

MainWindow::MainWindow(QWidget *parent) : QWidget(parent), ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    this->setStyleSheet("background-color: black;");
}

On my Mac, my whole application window is black (except the title bar).

EDIT : according to comment, here is a solution without using ui files and loading an external style sheet

#include <QtGui/QApplication>
#include <QtGui/QMainWindow>
#include <QtGui/QVBoxLayout>
#include <QtGui/QPushButton>
#include <QtCore/QFile>

int main(int ArgC, char* ArgV[])
{
QApplication MyApp(ArgC, ArgV);

QMainWindow* pWindow = new QMainWindow;
QVBoxLayout* pLayout = new QVBoxLayout(pWindow);
pWindow->setLayout(pLayout);

QPushButton* pButton = new QPushButton("Test", pWindow);
pLayout->addWidget(pButton);

QFile file(":/qss/default.qss");
file.open(QFile::ReadOnly);
QString styleSheet = QLatin1String(file.readAll());

qApp->setStyleSheet(styleSheet);

pWindow->setVisible(true);
MyApp.exec();
}

The style sheet file (default.qss) is as follow :

QWidget {
  background-color: black;
}

This file is part of a resource file (stylesheet.qrc) :

<RCC>
  <qresource prefix="/qss">
    <file>default.qss</file>
  </qresource>
</RCC>

And here is my project file :

TARGET = StyleSheet
TEMPLATE = app
SOURCES += main.cpp
RESOURCES += stylesheet.qrc