[ownedbycats]

To test, I changed the lines to

    def set_text_background_color(self):
        if not QApplication.instance().is_dark_theme:
            self.plain_qtextedit.setTextBackgroundColor(color_white)
            self.markdown_qtextedit.setTextBackgroundColor(color_white)
            self.html_qtextbrowser.setTextBackgroundColor(color_white)

It fixed it

EDIT: wrong screenshot, hang on

EDIT: Replaced screenshot

ANOTHER EDIT: Additional testing: It also works as expected using Calibre in System default with Windows in dark mode.