How does the Fusion style create 2 adjacent border colors?

Esor

I am trying to replicate this effect in QSS (rather than using QPalette). How can I do this? Any CSS solution seems to use box-shadow or :before/:after which aren't supported by QSS.

from PyQt5.QtWidgets import *
from PyQt5.QtGui import *
from PyQt5.QtCore import *

app = QApplication([])
button = QPushButton("Button")
layout = QVBoxLayout()
layout.addWidget(button)
container = QWidget()
container.show()
container.setLayout(layout)
button.setFixedSize(QSize(100, 30))
container.setFixedSize(QSize(500, 500))

darkPalette = QPalette()
darkPalette.setColor(QPalette.Button, QColor(53, 53, 53))
darkPalette.setColor(QPalette.ButtonText, QColor(180, 180, 180))
darkPalette.setColor(QPalette.Base, QColor(42, 42, 42))
darkPalette.setColor(QPalette.Window, QColor(53, 53, 53))

app.setStyle('Fusion')
app.setPalette(darkPalette)
app.exec_()