filter_youtube_previews.js

// ==UserScript==
// @name         Grayscale Youtube Previews
// @downloadURL  https://gist.githubusercontent.com/akatopo/4416756e8ca7a84098d6ab04dbc5dd71/raw/filter_youtube_previews.js
// @version      0.3
// @description  apply grayscale filter to youtube video previews
// @author       Alex Katopodis
// @match        https://www.youtube.com/*
// ==/UserScript==

(() => {
  'use strict';

  addGlobalStyle(`
    .ytd-grid-renderer,
    ytd-rich-grid-video-renderer,
    yt-horizontal-list-renderer,
    ytd-expanded-shelf-contents-renderer,
    ytd-watch-next-secondary-results-renderer {
      filter: grayscale(1);
    }
  `);

  function addGlobalStyle(css) {
    const head = document.getElementsByTagName('head')[0];
    if (!head) { 
      return;
    }
    const style = document.createElement('style');
    style.type = 'text/css';
    style.innerHTML = css;
    head.appendChild(style);
  }
})();