ニコニコ動画の視聴ページをいい感じにするやつ - At your own risk.

niconico.css

/* ==UserStyle==
@name           www.nicovideo.jp/watch
@namespace      github.com/openstyles/stylus
@version        1.0.0
@description    A new userstyle
@author         Me
==/UserStyle== */
@-moz-document url-prefix("https://www.nicovideo.jp/watch") {
    [aria-label="nicovideo-content"] > section {
        grid-template-areas: "player player" "meta sidebar" "bottom sidebar" ". sidebar";

        grid-template-columns: 1fr var(--sizes-watch-sidebar-width);
    }

    .grid-area_\[bottom\],
    .grid-area_\[meta\] {
        /* temp fix */
        width: calc(100vw - var(--sizes-watch-sidebar-width) - var(--spacing-x3) * 4);
    }

    .grid-area_\[player\] > div > div {
        margin: 0 auto;
        width: calc(var(--watch-player-height-by-browser) * 1.77777777778);        
        max-width: var(--watch-player-possible-width);
    }

    /* Player setting panel fix */
    .grid-area_\[sidebar\] {
        position: relative;
    }

    [aria-label="プレーヤー設定"] {
        position: absolute !important;
        top: 0 !important;
        left: auto !important;
        right: 0;
    }
}