Violentmonkey, mark watched YouTube videos

userscript.js

// ==UserScript==
// @name Mark watched YouTube videos
// @version 5
// @match https://www.youtube.com/feed/subscriptions
// @description Mark watched YouTube videos more prominently.
// @namespace https://gist.github.com/Siilwyn/
// @homepageURL https://gist.github.com/Siilwyn/669bb2b69cf039a02b6f209415a312f9/
// @downloadURL https://gist.githubusercontent.com/Siilwyn/669bb2b69cf039a02b6f209415a312f9/raw/userscript.js
// @updateURL   https://gist.githubusercontent.com/Siilwyn/669bb2b69cf039a02b6f209415a312f9/raw/userscript.js
// @grant GM_addStyle
// ==/UserScript==

GM_addStyle(`
  ytd-thumbnail-overlay-resume-playback-renderer {
    height: 100% !important;
    opacity: 0.7 !important;
  }

  ytd-thumbnail-overlay-resume-playback-renderer {
    background: none;
  }

  .ytd-thumbnail-overlay-resume-playback-renderer {
    height: 100% !important;
    background-color: #000000 !important;
  }
`);