How to change empty editor color?

[Borneq]

MainWindow::MainWindow() {
    createMenus();
    tabWidget = new QTabWidget;
    edit = new CodeEditor();
    //tabWidget->addTab(edit, "1");
    setCentralWidget(tabWidget);
}

MainWindow::MainWindow() {
    createMenus();
    tabWidget = new QTabWidget;
    edit = new CodeEditor();
    //tabWidget->addTab(edit, "1");
    setCentralWidget(tabWidget);
}

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If I write "tabWidget->add", is OK, one tab.
If not, is empty editor. Its color is white, I prefer gray and don't know, I must change MainWindow background color or empty QTabWidget background color (which is central widget)