The following stylesheet should place the QTableViews headers text in the middle and also the QTableWidgetItem.

QHeaderView::section {
text-align: left center;
}
QTableWidgetItem {
text-align: left center;
}

QTableWidgetItem's setTextAlignment should be exposed to qss:

QTableWidgetItem *sizeItem = new QTableWidgetItem(tr("%1 KB").arg(int((size + 1023) / 1024)));
sizeItem->setTextAlignment(Qt::AlignLeft | Qt::AlignVCenter);

QTableWidgets (and QTableViews) headers alignment can be also set in c++ with setDefaultAlignment() and it should be exposed to qss.

Please check the attached application for test.