Put inside firefox/$profile/chrome directory

userChrome.css

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");

/* to hide the native tabs */
#TabsToolbar {
    visibility: collapse;
}

/* to hide the sidebar header */
#sidebar-header {
    visibility: collapse;
}

@-moz-document url("about:newtab") {  
    body {
        background-color: #000000 !important;
    }
}

userContent.css

:root{
    scrollbar-color: rgb(0, 136, 216) rgb(30, 33, 34) !important;
}