Some css files to customize Firefox internal UI (place at $(user-profile-dir)/chrome/)

userChrome.css

/* 
 * darkern loading page background
 */
#browser vbox#appcontent tabbrowser,
#content,
#tabbrowser-tabpanels,
browser[type=content-primary],
browser[type=content] > html {
    background: black !important
}

#main-window,
browser[type="content-primary"],
browser[type="content"],
tabbrowser#content,
#content,
browser[type="content"] > html
{
    background: black !important;
}

userContent.css

/* 
 * darkern reader view sidebar
 */
@-moz-document url-prefix("about:reader") {
    .toolbar {
        background-color: #222 !important;
        border-right: 1px solid #555 !important;
    }

    .button {
        color: #cecece !important;
        background-color: #222 !important;
    }

    .toolbar .button {
        border-right: 1px solid #555 !important;
        border-bottom: 1px solid #555 !important;
    }
}

/* 
 * darkern new tab page background
 */
@-moz-document url("about:newtab") {
    body {
        background-color: #141c24 !important;
    }
}