/* Change background color for both private and non-private windows */
@-moz-document url("chrome://browser/content/browser.xhtml") {
    /* Non-private window background color */
    #appcontent, #appcontent tabpanels, browser[type="content-primary"], browser[type="content"] > html, browser[type="content"] > html > body {
        background-color: black !important;
    }

    /* Private window background color */
    :root[privatebrowsingmode="temporary"] #appcontent, :root[privatebrowsingmode="temporary"] #appcontent tabpanels, :root[privatebrowsingmode="temporary"] browser[type="content-primary"], :root[privatebrowsingmode="temporary"] browser[type="content"] > html, :root[privatebrowsingmode="temporary"] browser[type="content"] > html > body {
        background-color: black !important;
    }
}