hi , u can use this
QCalendarWidget QToolButton {
height: 60px;
width: 150px;
color: white;
font-size: 24px;
icon-size: 56px, 56px;
background-color: qlineargradient(x1:0, y1:0, x2:0, y2:1, stop: 0 #cccccc, stop: 1 #333333);
}
QCalendarWidget QMenu {
width: 150px;
left: 20px;
color: white;
font-size: 18px;
background-color: rgb(100, 100, 100);
}
QCalendarWidget QSpinBox {
width: 150px;
font-size:24px;
color: white;
background-color: qlineargradient(x1:0, y1:0, x2:0, y2:1, stop: 0 #cccccc, stop: 1 #333333);
selection-background-color: rgb(136, 136, 136);
selection-color: rgb(255, 255, 255);
}
QCalendarWidget QSpinBox::up-button { subcontrol-origin: border; subcontrol-position: top right; width:65px; }
QCalendarWidget QSpinBox::down-button {subcontrol-origin: border; subcontrol-position: bottom right; width:65px;}
QCalendarWidget QSpinBox::up-arrow { width:56px; height:56px; }
QCalendarWidget QSpinBox::down-arrow { width:56px; height:56px; }

/* header row */
QCalendarWidget QWidget { alternate-background-color: rgb(128, 128, 128); }

/* normal days */
QCalendarWidget QAbstractItemView:enabled
{
font-size:24px;
color: rgb(180, 180, 180);
background-color: black;
selection-background-color: rgb(64, 64, 64);
selection-color: rgb(0, 255, 0);
}

/* days in other months */
/* navigation bar */
QCalendarWidget QWidget#qt_calendar_navigationbar
{
background-color: qlineargradient(x1:0, y1:0, x2:0, y2:1, stop: 0 #cccccc, stop: 1 #333333);
}

QCalendarWidget QAbstractItemView:disabled
{
color: rgb(64, 64, 64);
}