样式分布图：

知道了每个 widget 后，就可以像下面这样用 QSS 修改 QCalendarWidget 的样式了。
示例1：

#qt_calendar_calendarview {
    background: white;
}

#qt_calendar_navigationbar {
    background: rgba(215, 215, 215, 255);
}

QToolButton {
    icon-size: 30px, 30px;
    width: 80px;
    height: 30px;
}

QToolButton#qt_calendar_prevmonth {
    background: green;
    width: 40px;
    qproperty-icon: url(:/img/left.png);
}

QToolButton#qt_calendar_nextmonth {
    background: blue;
    width: 40px;
    qproperty-icon: url(:/img/right.png);
}

QToolButton#qt_calendar_monthbutton {
    background: yellow;
    padding-right: 20px;
}

QToolButton#qt_calendar_yearbutton {
    background: magenta;
}

QToolButton#qt_calendar_monthbutton::menu-indicator{
    subcontrol-origin: padding;
    subcontrol-position: center right;
    right: 3px;
    width: 10px;
}

QAbstractItemView {
    color: black;
    selection-color: white;
    selection-background-color: rgb(255, 174, 0);
    font: 15px;
}

示例2

QCalendarWidget
{
border-radius:8px;
}
QWidget#qt_calendar_navigationbar{
min-height:50px;
}
QToolButton#qt_calendar_prevmonth {
    width: 40px;
    qproperty-icon: url(:/img/arrow_l.png);
}
QToolButton#qt_calendar_nextmonth {
    width: 40px;
    qproperty-icon: url(:/img/arrow_r.png);
}
QToolButton#qt_calendar_monthbutton {width:60px;}
QToolButton#qt_calendar_yearbutton {width:60px;}
QCalendarWidget QTableView
{
alternate-background-color: rgb(128, 128, 128);
background-color: #AFAFAE;
color:white;
selection-background-color:#dddddd;
selection-color:yellow;
border-left: 2px solid #AFAFAE;
border-right: 2px solid #AFAFAE;
border-bottom: 2px solid #AFAFAE;
border-bottom-left-radius:8px;border-bottom-right-radius:8px;
}

Qt源码：
以上QCalendarWidget各按键名称都是参考Qt源码的。

QCalendarWidget::QCalendarWidget(QWidget *parent)
    : QWidget(*new QCalendarWidgetPrivate, parent, 0)
{
    Q_D(QCalendarWidget);

    setAutoFillBackground(true);
    setBackgroundRole(QPalette::Window);

    QVBoxLayout *layoutV = new QVBoxLayout(this);
    layoutV->setMargin(0);
    d->m_model = new QCalendarModel(this);
    QTextCharFormat fmt;
    fmt.setForeground(QBrush(Qt::red));
    d->m_model->m_dayFormats.insert(Qt::Saturday, fmt);
    d->m_model->m_dayFormats.insert(Qt::Sunday, fmt);
    d->m_view = new QCalendarView(this);
    d->m_view->setObjectName(QLatin1String("qt_calendar_calendarview"));
    d->m_view->setModel(d->m_model);
    d->m_model->setView(d->m_view);
    d->m_view->setSelectionBehavior(QAbstractItemView::SelectItems);
    d->m_view->setSelectionMode(QAbstractItemView::SingleSelection);
    d->m_view->horizontalHeader()->setSectionResizeMode(QHeaderView::Stretch);
    d->m_view->horizontalHeader()->setSectionsClickable(false);
    d->m_view->verticalHeader()->setSectionResizeMode(QHeaderView::Stretch);
    d->m_view->verticalHeader()->setSectionsClickable(false);
    d->m_selection = d->m_view->selectionModel();
    d->createNavigationBar(this);
    d->m_view->setFrameStyle(QFrame::NoFrame);
    d->m_delegate = new QCalendarDelegate(d, this);
    d->m_view->setItemDelegate(d->m_delegate);
    d->update();
    d->updateNavigationBar();
    setFocusPolicy(Qt::StrongFocus);
    setFocusProxy(d->m_view);
    setSizePolicy(QSizePolicy::Preferred, QSizePolicy::Preferred);

    connect(d->m_view, SIGNAL(showDate(QDate)),
            this, SLOT(_q_slotShowDate(QDate)));
    connect(d->m_view, SIGNAL(changeDate(QDate,bool)),
            this, SLOT(_q_slotChangeDate(QDate,bool)));
    connect(d->m_view, SIGNAL(clicked(QDate)),
            this, SIGNAL(clicked(QDate)));
    connect(d->m_view, SIGNAL(editingFinished()),
            this, SLOT(_q_editingFinished()));

    connect(d->prevMonth, SIGNAL(clicked(bool)),
            this, SLOT(_q_prevMonthClicked()));
    connect(d->nextMonth, SIGNAL(clicked(bool)),
            this, SLOT(_q_nextMonthClicked()));
    connect(d->yearButton, SIGNAL(clicked(bool)),
            this, SLOT(_q_yearClicked()));
    connect(d->monthMenu, SIGNAL(triggered(QAction*)),
            this, SLOT(_q_monthChanged(QAction*)));
    connect(d->yearEdit, SIGNAL(editingFinished()),
            this, SLOT(_q_yearEditingFinished()));

    layoutV->setMargin(0);
    layoutV->setSpacing(0);
    layoutV->addWidget(d->navBarBackground);
    layoutV->addWidget(d->m_view);

    d->m_navigator = new QCalendarTextNavigator(this);
    setDateEditEnabled(true);
}
void QCalendarWidgetPrivate::createNavigationBar(QWidget *widget)
{
    Q_Q(QCalendarWidget);
    navBarBackground = new QWidget(widget);
    navBarBackground->setObjectName(QLatin1String("qt_calendar_navigationbar"));
    navBarBackground->setAutoFillBackground(true);
    navBarBackground->setBackgroundRole(QPalette::Highlight);

    prevMonth = new QPrevNextCalButton(navBarBackground);
    nextMonth = new QPrevNextCalButton(navBarBackground);
    prevMonth->setAutoRaise(true);
    nextMonth->setAutoRaise(true);
    prevMonth->setSizePolicy(QSizePolicy::Fixed, QSizePolicy::Minimum);
    nextMonth->setSizePolicy(QSizePolicy::Fixed, QSizePolicy::Minimum);
    nextMonth->setAutoRaise(true);
    updateButtonIcons();
    prevMonth->setAutoRepeat(true);
    nextMonth->setAutoRepeat(true);

    monthButton = new QCalToolButton(navBarBackground);
    monthButton->setSizePolicy(QSizePolicy::Fixed, QSizePolicy::Minimum);
    monthButton->setAutoRaise(true);
    monthButton->setPopupMode(QToolButton::InstantPopup);
    monthMenu = new QMenu(monthButton);
    for (int i = 1; i <= 12; i++) {
        QString monthName(q->locale().standaloneMonthName(i, QLocale::LongFormat));
        QAction *act = monthMenu->addAction(monthName);
        act->setData(i);
        monthToAction[i] = act;
    }
    monthButton->setMenu(monthMenu);
    yearButton = new QCalToolButton(navBarBackground);
    yearButton->setSizePolicy(QSizePolicy::Fixed, QSizePolicy::Minimum);
    yearButton->setAutoRaise(true);
    yearEdit = new QSpinBox(navBarBackground);

    QFont font = q->font();
    font.setBold(true);
    monthButton->setFont(font);
    yearButton->setFont(font);
    yearEdit->setFrame(false);
    yearEdit->setMinimum(m_model->m_minimumDate.year());
    yearEdit->setMaximum(m_model->m_maximumDate.year());
    yearEdit->hide();
    spaceHolder = new QSpacerItem(0,0);

    QHBoxLayout *headerLayout = new QHBoxLayout;
    headerLayout->setMargin(0);
    headerLayout->setSpacing(0);
    headerLayout->addWidget(prevMonth);
    headerLayout->insertStretch(headerLayout->count());
    headerLayout->addWidget(monthButton);
    headerLayout->addItem(spaceHolder);
    headerLayout->addWidget(yearButton);
    headerLayout->insertStretch(headerLayout->count());
    headerLayout->addWidget(nextMonth);
    navBarBackground->setLayout(headerLayout);

    yearEdit->setFocusPolicy(Qt::StrongFocus);
    prevMonth->setFocusPolicy(Qt::NoFocus);
    nextMonth->setFocusPolicy(Qt::NoFocus);
    yearButton->setFocusPolicy(Qt::NoFocus);
    monthButton->setFocusPolicy(Qt::NoFocus);

    //set names for the header controls.
    prevMonth->setObjectName(QLatin1String("qt_calendar_prevmonth"));
    nextMonth->setObjectName(QLatin1String("qt_calendar_nextmonth"));
    monthButton->setObjectName(QLatin1String("qt_calendar_monthbutton"));
    yearButton->setObjectName(QLatin1String("qt_calendar_yearbutton"));
    yearEdit->setObjectName(QLatin1String("qt_calendar_yearedit"));

    updateMonthMenu();
    showMonth(m_model->m_date.year(), m_model->m_date.month());
}