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.
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.