My button's color seems transparent in QtDesigner but why?

[bladekel]

I'm using QTDesigner 5.15.8 and trying to color buttons.

color.png

I have inserted one button and frame on my mainwindow. Here is the stylesheet code of mainwindow

QPushButton{
background-color: red;
}
QFrame{
background-color: red;
}

QPushButton{
background-color: red;
}
QFrame{
background-color: red;
}

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They are same but as you see they look different.

I couldnt find why it happened but it happens even if I created a new project...