Raw Source
OnTake / Youtube Custom Theme v2 

// ==UserScript==
// @name         Youtube Custom Theme v2
// @namespace    http://www.youtube.com/
// @version      0.18
// @description  Custom Youtube Theme
// @author       Ontake
// @match        https://www.youtube.com/*
// @grant        none
// @copyright 2020, OnTake (https://openuserjs.org/users/OnTake)
// @license MIT
// ==/UserScript==

(function() {
    'use strict';

    var csselement = document.createElement("style");
        csselement.innerHTML = "#guide-inner-content::-webkit-scrollbar-thumb {background-color: #ffffff10;}#guide-inner-content::-webkit-scrollbar{width: 0.1vw;}body::-webkit-scrollbar{width: 0.1vw;}body::-webkit-scrollbar-thumb {background-color: #ffffff10;}.yt-icon-container{transition:filter 2s ease;filter:hue-rotate(0deg);}.yt-icon-container:hover{filter:hue-rotate(360deg);}.video-stream{border-radius:1vw;}.ytd-app{border-radius:1vw;}paper-button.ytd-subscribe-button-renderer{border-radius:100vw;}paper-button.ytd-subscribe-button-renderer {border-radius:100vw;}#container.ytd-masthead{border-radius: 0vw 0vw 1vw 1vw;}ytd-masthead#masthead{border-radius: 0vw 0vw 1vw 1vw;}#tabs-inner-container.ytd-c4-tabbed-header-renderer{background-color:#FFFFFF11;}#tabs-container.ytd-c4-tabbed-header-renderer{background-color:transparent;}#channel-header.ytd-c4-tabbed-header-renderer{background-color:transparent;}img.yt-img-shadow{border-radius:0.3vw;}#container.ytd-searchbox {border-radius: 100vw 0vw 0vw 100vw;}#search-icon-legacy.ytd-searchbox {border-radius: 0vw 100vw 100vw 0vw;}ytd-thumbnail-overlay-time-status-renderer {border-radius:1vw;}.html5-video-player {border-radius:1vw;} ytd-button-renderer#button.ytd-button-renderer {border-radius:100vw;}.ytp-settings-menu{border-radius:1vw;}ytd-multi-page-menu-renderer {border-radius:1vw;}.caption-visual-line{border-radius: 1vw;}.ytp-caption-segment {border-radius: 1vw;}ytd-button-renderer.style-suggestive[is-paper-button] paper-button.ytd-button-renderer {border-radius:100vw;}#container.ytd-searchbox {transition: border 0.1s ease;}iron-dropdown{animation: dropdown 0.3s ease;} @keyframes dropdown{from{opacity:0;}to{opacity:1;}}";
        document.body.appendChild(csselement);
})();