This repository contains two files that help make the YouTube player look like 2017 again!
For yt_player_classicifier.user.css, use Stylus (Firefox, Chromium).
For old_seek_ui.user.js, use Tampermonkey (Firefox, Chromium).
With both of these files, click them on GitHub and click the "Raw" button at the top of the code viewer. This should prompt the respective extensions to install these files.