If you are using Firefox and YouTube seems slow to load you can speed it up (up to 5x) with this tweak in about:config.
-Open new tab and enter about:config (accept if prompted for warranty risk)
-Right click on any preference and go New>String
>As preference name copy/paste:
general.useragent.override.youtube.com
As preference value copy/paste:
Mozilla/5.0 (Windows NT 6.1; WOW64; rv:41.0) Gecko/20100101 Firefox/41.0
For darkmode you can install Stylus addon and pick theme you like.