How to change border color only and not change other of QComboBox?

[vinh]

I have QComboBox: Screenshot_1.png
I check no change item and set border color red: Screenshot_2.png
my code change border color:

combobox->setStyleSheet( combobox->styleSheet().append( QString( "border-width: 1px; border-style: solid; border-color: red;" ) ) );

combobox->setStyleSheet( combobox->styleSheet().append( QString( "border-width: 1px; border-style: solid; border-color: red;" ) ) );

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size change too, why? How to change boder color only?