user-style.css

@-moz-document domain("github.com") {
    *,*:before,*:after {
        background-color: inherit !important;
        background-image: none !important;
        text-shadow: none !important;
        box-shadow: none !important;
        border-color: #bbb !important;
    }

    html,body {
        background: #aaa !important;
    }

    pre,code,svg,.menu,#readme {
        background: #ccc !important;
    }

    span,a {
        background-color: transparent !important;
    }

    a {
        color: #369 !important;
    }

    input[type=text],textarea {
        background: #eee !important;
    }

    .modal-backdrop {
        background-color: transparent !important;
    }
}