userContent.css

@-moz-document regexp("about:(newtab|home)") {
    body {
        background-color: #222 !important;
        overflow-y:hidden!important;
    }
    .prefs-pane-button button{
        fill: rgba(255, 255, 255, 0.6)!important;
    }
    .prefs-pane-button button:hover{
        background-color:#222!important;
        fill: rgba(255, 255, 255, 0.3)!important;
    }
    .sidebar {
        background-color: #333!important;
        color: #FFF!important;
        border-left: none!important;
    }
    .sidebar .actions {
        border: none!important;
        background-color: #444!important;
        color: #FFF!important;
    }
    .sidebar .actions .done {
        background: #383838!important;
        border: none!important;
    }
    .sidebar .actions button:hover:not(.dismiss) {
      box-shadow: 0px 0px 5px 2px rgba(255, 255, 255, 0.3)!important;
      transition: box-shadow 150ms!important;
    }
    #onboarding-overlay-button {
        display: none!important;
    }
}