Comment by thamer

The following CSS equivalent worked for me, using the "Custom CSS by Denis" Chrome extension[1]:

    ytd-rich-grid-renderer div#contents {
      /* number of video thumbnails per row */
      --ytd-rich-grid-items-per-row: 5 !important;
    
      /* number of Shorts per row in its dedicated section */
      --ytd-rich-grid-slim-items-per-row: 6 !important;
    }

I first tried it with the "User JavaScript and CSS" extension, but somehow it didn't seem able to inject CSS on YouTube. Even a simple `html { border: 5px solid red; }` would not show anything, while I could see it being applied immediately with the "Denis" CSS extension.

If someone can recommend a better alternative for custom CSS, I'd be interested to hear it. I guess Tampermonkey could work, if you have that.

[1] https://chromewebstore.google.com/detail/custom-css-by-denis...