Try YouTube's Dark Theme, works only with Google Chrome.
1. Go to YouTube.com and log into your account.
2.Open Chrome's Developer Tools panel by hitting Command-Option-I on a Mac or Ctrl-Shift-I on Windows.
3.On the right side of your Chrome window, click the Console tab.
4.Copy and paste this line of code: document.cookie="VISITOR_INFO1_LIVE=fPQ4jCL6EiE"
5.and then hit Enter.
6.Refresh The Page, Click Your Picture in Upper Right Hand Corner of Youtube AND IF it worked u will see An "Dark Mode" Option Appear now.