blacktheme.css

div#container.style-scope.ytd-masthead, 
ytd-browse.style-scope.ytd-page-manager,
div#sections.style-scope.ytd-guide-renderer,
div#footer.style-scope.ytd-guide-renderer,
div#header.style-scope.ytd-multi-page-menu-renderer,
ytd-active-account-header-renderer.style-scope.ytd-multi-page-menu-renderer,
ytd-simple-menu-header-renderer.style-scope.ytd-multi-page-menu-renderer,
ytd-multi-page-menu-renderer.dropdown-content.style-scope.ytd-popup-container
div#sections.style-scope.ytd-multi-page-menu-renderer,
ytd-post-renderer.style-scope.yt-horizontal-list-renderer,
div#columns.style-scope.ytd-watch-flexy,
ytd-watch-flexy.style-scope.ytd-page-manager.hide-skeleton,
ytd-playlist-sidebar-renderer.style-scope.ytd-browse,
div#header.style-scope.ytd-app,
.header-bar,
.scbrr-tabs,
lazy-list,
.menu-content {
    background: black;
}

.scbrr-tabs,
lazy-list,
.menu-content {
    color: white;
}