You will need to install a user script manager extension to install this script.
A low-tech solution to a high-tech problem! Automatically clicks YouTube's "Dark Mode" button if dark mode isn't already active.
You must already have YouTube's material theme (you can opt-in at http://youtube.com/new). This script automates the job of enabling "Dark Mode" when necessary.
Every time I reset my browser's cache/cookies, I was faced with a white YouTube theme again, and I was tired of switching to Dark Mode manually. This script automates it in a pretty funny way: It automatically opens the Settings menu and clicks the Dark Mode button. Why such a low-tech solution? Because YouTube's page is very advanced and doesn't easily allow 3rd party scripts to switch its settings. There is a more advanced technique commented-out in this code, which actually uses YouTube's internal Event system to signal the change, but I disabled that code because it was much less reliable than the low-tech solution. It's still in this script's source code if anyone is curious, though.
This script is fully compatible with my similar script, Automatic Privacy Reminder Killer for YouTube.
Totally optional. ;-)
The script hardcodes the name and location of the menu items it clicks, so there's a very big chance that it'll stop working within a few years (whenever YouTube changes their design again). If it breaks, do NOT freak out and blast this page with negative reviews. Instead, be patient and wait while I update it when I have time. If you ungratefully blast me with negative reviews due to YouTube suddenly changing their design (which isn't my fault), then I won't bother updating this script for you at all.