Стилизация текста в QTableView с PySide2

Вы должны использовать делегата, который использует QTextDocument:

import random
from PySide2 import QtCore, QtGui, QtWidgets

words = '''Lorem ipsum dolor sit amet, consectetur adipiscing elit. 
Mauris euismod cursus mi sit amet pellentesque. 
Proin sed lectus sed augue scelerisque congue eget quis leo. 
Curabitur ultrices nisi nisi, placerat gravida urna sagittis et. 
Nullam vitae urna tortor. Curabitur a lobortis metus, et laoreet arcu. 
Quisque a mi in purus molestie porta non sit amet purus. 
Sed porta non purus et suscipit.'''.split()

class HighlightDelegate(QtWidgets.QStyledItemDelegate):
    def __init__(self, parent=None):
        super(HighlightDelegate, self).__init__(parent)
        self.doc = QtGui.QTextDocument(self)
        self._regex = QtCore.QRegularExpression()
        self._highlight_format = QtGui.QTextCharFormat()

    def paint(self, painter, option, index):
        painter.save()
        options = QtWidgets.QStyleOptionViewItem(option)
        self.initStyleOption(options, index)
        self.doc.setPlainText(options.text)
        self.apply_highlight()
        options.text = ""
        style = QtWidgets.QApplication.style() if options.widget is None \
            else options.widget.style()
        style.drawControl(QtWidgets.QStyle.CE_ItemViewItem, options, painter)

        ctx = QtGui.QAbstractTextDocumentLayout.PaintContext()
        if option.state & QtWidgets.QStyle.State_Selected:
            ctx.palette.setColor(QtGui.QPalette.Text, option.palette.color(
                QtGui.QPalette.Active, QtGui.QPalette.HighlightedText))
        else:
            ctx.palette.setColor(QtGui.QPalette.Text, option.palette.color(
                QtGui.QPalette.Active, QtGui.QPalette.Text))

        textRect = style.subElementRect(
            QtWidgets.QStyle.SE_ItemViewItemText, options)

        if index.column() != 0:
            textRect.adjust(5, 0, 0, 0)

        the_constant = 4
        margin = (option.rect.height() - options.fontMetrics.height()) // 2
        margin = margin - the_constant
        textRect.setTop(textRect.top() + margin)

        painter.translate(textRect.topLeft())
        painter.setClipRect(textRect.translated(-textRect.topLeft()))
        self.doc.documentLayout().draw(painter, ctx)

        painter.restore()

    def apply_highlight(self):
        cursor = QtGui.QTextCursor(self.doc)
        cursor.beginEditBlock()
        highlightCursor = QtGui.QTextCursor(self.doc)
        while not highlightCursor.isNull() and not highlightCursor.atEnd():
            highlightCursor = self.doc.find(self.regex, highlightCursor)
            if not highlightCursor.isNull():
                highlightCursor.mergeCharFormat(self.highlightFormat)
        cursor.endEditBlock()

    @property
    def regex(self):
        return self._regex

    @regex.setter
    def regex(self, regex):
        if self._regex == regex: return
        self._regex = regex

    @property
    def highlightFormat(self):
        return self._highlight_format

    @highlightFormat.setter
    def highlightFormat(self, fmt):
        self._highlight_format = fmt

class MainWindow(QtWidgets.QMainWindow):
    def __init__(self, parent=None):
        super(MainWindow, self).__init__(parent)

        self.table = QtWidgets.QTableView()

        self._delegate = HighlightDelegate(self.table)
        self._delegate.regex = QtCore.QRegularExpression(r"\(.*?\)")
        fmt = QtGui.QTextCharFormat()
        fmt.setForeground(QtCore.Qt.green)
        fmt.setFontWeight(QtGui.QFont.Bold)
        self._delegate.highlightFormat = fmt
        self.table.setItemDelegate(self._delegate)

        model = QtGui.QStandardItemModel(10, 4)
        for i in range(model.rowCount()):
            for j in range(model.columnCount()):
                item = QtGui.QStandardItem("{}({}){}".format(*random.sample(words,3)))
                model.setItem(i, j, item)
        self.table.setModel(model)
        self.setCentralWidget(self.table)

if __name__ == '__main__':
    import sys
    app = QtWidgets.QApplication(sys.argv)
    w = MainWindow()
    w.show()
    sys.exit(app.exec_())