You could just add set the layout on an empty QWidget and set the StyleSheet on this widget.
for index, param_name in enumerate(parameters):
container = QtGui.QWidget(self)
layout = QtGui.QHBoxLayout(container )
hlayouts.append(container)
labels.append(QtGui.QLabel("%s"%param_name))
sliders.append(QtGui.QSpacerItem(10,10,hPolicy=QtGui.QSizePolicy.Expanding))
spins.append(QtGui.QDoubleSpinBox())
spins[index].setValue(float(values.get(param_name)))
container.setStyleSheet("background-color:black;")
layout.addWidget(labels[index])
layout.addItem(sliders[index])
layout.addWidget(spins[index])
vlayout = QtGui.QVBoxLayout(self)
for widget in hlayouts:
vlayout.addWidget(widget)