Qt Stylesheet for custom widget

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.

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
....

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)

I had same problem with pyside. I post my solution just for completeness. It is almost like in PyQt as Pieter-Jan Busschaert proposed. only difference is you need to call initFrom instead of init

def paintEvent(self, evt):
    super(FreeDockWidget,self).paintEvent(evt)
    opt = QtGui.QStyleOption()
    opt.initFrom(self)
    p = QtGui.QPainter(self)
    s = self.style()
    s.drawPrimitive(QtGui.QStyle.PE_Widget, opt, p, self) 

One other thing you need to make sure is that you define your custom widget in your css file the following way:

FreeDockWidget{...}

and not like often recommended

QDockWidget#FreeDockWidget{...}