Qt Stylesheet for custom widget

For completeness, the same problem is present in PyQt. You can apply a stylesheet to a subclassed QWidget by adding similar code:

def paintEvent(self, pe):
  opt = QtGui.QStyleOption()
  opt.init(self)
  p = QtGui.QPainter(self)
  s = self.style()
  s.drawPrimitive(QtGui.QStyle.PE_Widget, opt, p, self)

There is an answer much easier than writing your own paintEvent: subclass QFrame instead of QWidget and it will work right away:

class WidgetUnits : public QFrame
{
    Q_OBJECT
....

I had a similar problem and it was solved using jecjackal's comment. As sjwarner said, it would be much more noticeable in the form of an answer. So I'll provide it. For the benefit of any future viewers. Again, it isn't my answer! Appreciate jecjackal for it!

As it is said in the Qt's stylesheets reference, applying CSS styles to custom widgets inherited from QWidget requires reimplementing paintEvent() in that way:

 void CustomWidget::paintEvent(QPaintEvent *)
 {
     QStyleOption opt;
     opt.init(this);
     QPainter p(this);
     style()->drawPrimitive(QStyle::PE_Widget, &opt, &p, this);
 }

Without doing it your custom widgets will support only the background, background-clip and background-origin properties.

You can read about it here: Qt Stylesheets reference in the section "List of Stylable Widgets" -> QWidget.