c++ - How to set background color of QComboBox button?

Question

Version/Environment:

• Windows 10 64 bit
• Qt 5.11.0 MSVC2017 64 bit

I have a simple QComboBox to enable/disable a feature:

QComboBox *onOffComboBox = new QComboBox();
onOffComboBox->insertItem(0, "Off");
onOffComboBox->insertItem(1, "On");

The combo box is added as a cell widget to a table:

this->ui->settingsTable->setCellWidget(rowNumber, 1, onOffComboBox);

Now i want to change the background color of the button but not the select items.

My first approach was simply to use QWidget's setStyleSheet function:

onOffComboBox->setStyleSheet("background-color: red;");

But this suppresses the standard style:

I also used variations with specific QComboBox styles according to the documentation:

onOffComboBox->setStyleSheet("QComboBox::drop-down {background: red;}");

But this only colors the part with the arrow and suppresses it's style:

Using just QComboBox {background: red;} has the same result as with background-color: red; just the select items are not colored.

Just as described in this answer another approach is to use QPallete:

QPalette pal = onOffComboBox->palette();
pal.setColor(QPalette::Base, QColor("red"));
onOffComboBox->setPalette(pal);
onOffComboBox->update(); // just in case this has any effect

This only colors the select items:

I also tried nearly all other QPalette color roles:

• QPalette::Window, QPalette::Foreground, QPalette::Button - do nothing
• QPalette::Base - colors the select items (see pic)
• QPalette::Text - colors the text of the button and the select items

So, how can i change the color of the QComboBox drop-down button background WITHOUT overwritting or suppressing the standard style?

The styles of the pop-up items also shouldn't change.

Here is an image of what i want:

Answer 1

QComboBox is always tricky to customize because it is made of subwidgets (even conditional subwidgets).
I made tests and your simple stylesheet QComboBox {background:red} works almost fine for me on Linux, except that the box-border is also red in the dropdown. The behaviour seems to be different depending on the GUI style.

From the doc :

Note: With complex widgets such as QComboBox and QScrollBar, if one property or sub-control is customized, all the other properties or sub-controls must be customized as well.

It looks like you have to customize everything if you want to customize the button... Then it would not be possible to change the colour without overwriting or suppressing the standard style.

You could style the whole drop-down to mimic the native look, but it's not nice to do and not robust (and not portable).

QComboBox QAbstractItemView {
  border: 1px solid grey;
  background: white;
  selection-background-color: blue;
}
QComboBox {
  background: red;
}