ニコニコ動画・生放送の視聴ページをダークテーマ化するユーザーCSS（Stylusを想定。コントラスト等を考慮して作成されたものではありません。ポップアップ等には非対応です）

nicodark.user.css

@-moz-document url-prefix("https://live.nicovideo.jp/watch") {
#watchPage
{
    background-color: #333;
    color: #ccc;
}

.___program-information-header-area___\+Y3b5,
.___program-information-area___9OIq6,
.___program-summary___uo308,
.___player-status-panel___dZMog,
.___broadcast-participation-area___Y7TbT,
.comment-text-box,
.ga-ns-promotion-panel,
.ga-ns-comment-panel,
.ng-setting-controller,
.tab-area,
.comment-tab,
.program-recommend-tab
{
    background-color: #333 !important;
    color: #ccc !important;
    box-shadow: none !important;
}

.___leo-player___3AVyh[data-fullscreen-layout] .___broadcast-participation-area___Y7TbT
{
    background-color: transparent !important;
}

.comment-panel>.data-grid>.body>.table>.table-row[data-comment-type=normal]>.table-cell
{
    --local-comment-text-color: #ccc;
}

.comment-panel>.data-grid>.body>.table>.table-row[data-comment-type=cruise]>.table-cell,
.comment-panel>.data-grid>.body>.table>.table-row[data-comment-type=programExtend]>.table-cell,
.comment-panel>.data-grid>.body>.table>.table-row[data-comment-type=quote]>.table-cell,
.comment-panel>.data-grid>.body>.table>.table-row[data-comment-type=ranking]>.table-cell,
.comment-panel>.data-grid>.body>.table>.table-row[data-comment-type=spi]>.table-cell,
.comment-panel>.data-grid>.body>.table>.table-row[data-comment-type=userFollow]>.table-cell,
.comment-panel>.data-grid>.body>.table>.table-row[data-comment-type=userLevelup]>.table-cell,
.comment-panel>.data-grid>.body>.table>.table-row[data-comment-type=visited]>.table-cell
{
    --local-comment-text-color: #999;
}

.ga-ns-program-information,
.ga-ns-description,
.ga-ns-program-summary,
.ga-ns-program-summary h1,
.ga-ns-program-summary time,
.creator-support-supporter-registration-section,
.ga-ns-social-group-information > div, /* コミュニティ説明文 */
.ga-ns-social-group-information a, /* コミュニティ名 */
.user-name, /* 放送者名 */
.ga-ns-user-program-broadcast-history-section h2, /* 最近の番組 */
.program-title, /* 最近の番組 タイトル */
.program-card, /* 最近の番組 背景 */
.ga-ns-user-program-broadcast-history-section a, /* 最近の番組 > もっと見る */
.ga-ns-konomi-tag-list-section, /* 放送者の好きなものリスト 背景 */
.ga-ns-konomi-tag-list-section h2, /* 放送者の好きなものリスト タイトル */
.ga-ns-related-content-panel h2,
.ga-ns-related-content-card .content-area,
.ga-ns-related-content-card .content-title
{
    background-color: #444 !important;
    color: #ccc !important;
}

}

@-moz-document url-prefix("https://www.nicovideo.jp/watch") {
#js-app
{
    background-color: #333;
    color: #ccc;
}

.WatchAppContainer,
.HeaderContainer,
.EasyCommentContainer,
.ControllerBoxCommentAreaContainer,
.CommentPostContainer-commandInput,
.CommentPostContainer-commentInput,
.VideoMenuContainer-areaLeft,
.VideoMenuContainer-areaRight,
.PlayerPanelContainer,
.CommentPanelContainer,
.BottomContainer,
.Card,
.IchibaForWatch
{
    background-color: #333;
    color: #ccc;
}

.CommentPanelContainer .CommentPanelDataGrid
{
    color: rgba(255, 255, 255, 0.5);
}

.CommentPanelDataGrid .CommentPanelDataGrid-HeaderCell {
    background-color: #333;
}

.CommentPanelContainer [data-row-nicoru-rank="1"]
{
    /* #fefbec */
    background-color: rgba(254, 251, 236, 0.5);
    color: rgba(0, 0, 0, 0.75);
}
.CommentPanelContainer [data-row-nicoru-rank="2"]
{
    /* #fef5cf */
    background-color: rgba(254, 245, 207, 0.5);
    color: rgba(0, 0, 0, 0.75);
}
.CommentPanelContainer [data-row-nicoru-rank="3"]
{
    /* #fdeba0 */
    background-color: rgba(253, 235, 160, 0.5);
    color: rgba(0, 0, 0, 0.75);
}
.CommentPanelContainer [data-row-nicoru-rank="4"]
{
    /* #fcd842 */
    background-color: rgba(252, 216, 66, 0.5);
    color: rgba(0, 0, 0, 0.75);
}

}