Where to find QSS files of the default QGIS UI theme?

Question

In QGIS 3.28.10-Firenze on my Windows, I am trying to find where the default QGIS UI theme with its files is located.

I could find both "Blend of Gray" and "Night Mapping" under this path C:\OSGeo4W\apps\qgis-ltr\resources\themes, albeit the default UI theme.

This directory C:\Users\taras\AppData\Roaming\QGIS\QGIS3\profiles\default\themes is empty.

My goal is to find files of the default UI theme: style.qss and variables.qss.

I tried the "Load QSS - UI themes" plugin, which was mentioned in this thread Change Layers Panel background color in QGIS. It has a Default button (Default_btn), however, it does refer to a specific file, rather of setting styles to an empty "" value.

# Restores style
def ResetStyle(self):
    self.app.setStyleSheet("")
    setActivated("")
    return

QGIS also has a defaultThemePath() variable inside the qgsapplication.cpp file, see QGIS repository on the GitHub. But it is mostly about images and SVG-files.

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References:

• QGIS: I can barely see the labels in layer panel