There was qtquickcontrols2.conf
in the root directory of my app sources with the following content:
; This file can be edited to change the style of the application
; See Styling Qt Quick Controls 2 in the documentation for details:
; https://doc.qt.io/qt-6/qtquickcontrols2-styles.html
[Controls]
;Style=Default
;Style=Universal
;Style=Material
Style=Fusion
;Style=iOS
[Universal]
Theme=Light
Accent=Steel
[Material]
;Theme=Light
Theme=Dark
;Accent=BlueGrey
;Primary=BlueGray