CSS Stylesheet is not applied to custom QWidget

Question

What I'm trying to do is to apply custom CSS to a custom widget deriving from QLabel but I'm having no luck.

I have the custom class defined as:

class CustomLabel : public QLabel {

}

I've not re-implemented the paintEvent function as I thought that given that the standard QLabel supports CSS stylesheets, I would just need to refer to this new widget in CSS, for example:

CustomLabel {
    background-color: #111111;
    font-size: 18px;
    font-weight: bold;
}

Unfortunately at run-time, no style is applied the CustomLabel and the default QLabel style is applied.

Can anyone suggest why my CSS rules are being ignored for CustomLabel?

Steps to Recreate

1. Create a Qt Widgets project using Qt Creator
2. Add a custom class derived from QLabel and call it CustomLabel
3. Add a QLabel onto a form using the designer
4. Promote the label to a CustomLabel class

5. Apply the stylesheet using the following code in main.cpp:

   a.setStyleSheet("CustomLabel {font-weight: bold;}");
   

6. Run the program and notice how CustomLabel is not styled in accordance with the CSS style.

Answer 1

You should use the macro Q_OBJECT inside your CustomLabel definition, otherwise CustomLabel is not known to Qt's type system:

class CustomLabel : public QLabel {
    Q_OBJECT
}

MCVE

CustomLabel.h:

#include "QLabel"
class CustomLabel : public QLabel {
  Q_OBJECT
};

main.cpp:

#include "QApplication"
#include "CustomLabel.h"
int main(int argc, char * argv[])
{
  QApplication a(argc, argv);
  a.setStyleSheet("CustomLabel {font-weight: bold; background-color: red;}");

  CustomLabel label;
  label.setText ("Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua");
  label.show ();

  a.exec ();
}

Answer 2

Try settings Qt::WA_StyledBackground attribute for your custom label.