A UserStyle for anyone using Tabview for YouTube: https://github.com/tabview-youtube/Tabview-YouTube-Totara/

you-r-tubluar.user.css

/* ==UserStyle==
@name           You(r)Tubular
@namespace      github.com/kckarnige
@version        1.0.0
@description    Makes the YouTube tabview UserScript fit in with YouTube's UI style.
@author         KiCKTheBucket
==/UserStyle== */

@-moz-document domain("youtube.com") {

    html[dark],
    html[dark]>body {
        color-scheme: dark;
    }

    [fill*="#FF0000"],
    [fill*="#F00"],
    [fill*="red"] {
        fill: var(--yt-spec-static-brand-red) !important;
    }

    .ytp-settings-button.ytp-hd-quality-badge:after,
    .ytp-settings-button.ytp-hdr-quality-badge:after,
    .ytp-settings-button.ytp-4k-quality-badge:after,
    .ytp-settings-button.ytp-5k-quality-badge:after,
    .ytp-settings-button.ytp-8k-quality-badge:after,
    .ytp-settings-button.ytp-3d-badge-grey:after,
    .ytp-settings-button.ytp-3d-badge:after,
    .ytp-swatch-background-color {
        background-color: var(--yt-spec-static-brand-red);
    }

    ytd-watch-flexy:not([theater]):not([fullscreen]):not([no-top-margin]):not([reduced-top-margin]) #secondary.ytd-watch-flexy {
        padding-top: 0px;
    }

    ytd-watch-flexy:not([theater]):not([fullscreen]):not([no-top-margin]):not([reduced-top-margin]) #primary.ytd-watch-flexy {
        padding-top: 0px;
    }

    ytd-subscribe-button-renderer .yt-spec-button-shape-next--mono.yt-spec-button-shape-next--filled {
        color: var(--yt-spec-static-brand-white);
        background-color: var(--yt-spec-static-brand-red);
    }

    ytd-subscribe-button-renderer .yt-spec-button-shape-next--mono.yt-spec-button-shape-next--filled:hover {
        color: var(--yt-spec-static-brand-white);
        background-color: var(--yt-spec-brand-button-background);
    }


    /* Tab Stuff */
    @supports (color: var(--tabview-tab-btn-define)) {

        html {
            --tyt-tab-btn-flex-grow: 1;
            --tyt-tab-btn-flex-basis: 0%;
            --tyt-tab-bar-color-1-def: var(--yt-brand-light-red);
            --tyt-tab-bar-color-2-def: var(--yt-spec-static-brand-red);
            --tyt-tab-bar-color-1: var(--main-color, var(--tyt-tab-bar-color-1-def));
            --tyt-tab-bar-color-2: var(--main-color, var(--tyt-tab-bar-color-2-def));
        }

        ytd-watch-flexy #right-tabs .tab-btn {
            background-color: var(--yt-spec-badge-chip-background);
        }

        ytd-watch-flexy #right-tabs .tab-content {
            background-color: var(--yt-spec-badge-chip-background);
        }

        ytd-watch-flexy #right-tabs .tab-btn[tyt-tab-content].active {
            font-weight: 500;
            outline: 0;
            --tyt-tab-btn-color: var(--yt-spec-text-primary);
            background-color: var(--yt-spec-outline) !important;
            border-bottom: 2px var(--tyt-tab-bar-color-1) solid;
            border-bottom-color: var(--tyt-tab-bar-color-2);
        }

        #right-tabs #material-tabs,
        ytd-watch-flexy #right-tabs .tab-content {
            border: none;
        }

        ytd-watch-flexy #right-tabs .tab-btn[tyt-tab-content] {
            background-color: var(--yt-spec-badge-chip-background) !important;
        }

        secondary-wrapper {
            padding-top: 0px;
        }

        ytd-watch-flexy #right-tabs .tab-content {
            border-radius: 0 0 12px 12px;
        }

        [tyt-tab^="#"] #right-tabs #material-tabs {
            border-radius: 12px 12px 0 0;
        }

        [tyt-tab^="#"] #right-tabs #material-tabs {
            border-radius: 12px 12px 0 0;
        }
    }
}

Requirements

uBlock Origin (Recommended) - Firefox - Chrome

Tampermonkey - Firefox - Chrome

Stylus - Firefox - Chrome

Tabview YouTube - UserScript Install

-> UserStyle Install <-