There is no definitive answer to this question. Some people like dark modes and shades of black, but some prefer the comfort of a traditional color layout. Also, this question can be a topic of a never ending debate, because each person may perceive this based on his personal opinion regarding the color schemes.
Notably, Gmail, Yahoo, and most of the mail based websites have an option for custom themes. This gives the power of the choice to the user, in such a way that he/she may customize their inbox or view the way the want it. This is a brilliant move, in terms of giving the user a personalized feeling.
Also, it is interesting to know that themes mostly are used in applications, rather than websites. This is because we often tend to spend a larger amount of time on applications, rather than websites, especially with the rise of usage of smartphones.
As far as websites are concerned, they often have a theme which is aligned to their brand and identity. Facebook is one such an example. People practically spend hours on the world's most popular social media channel. But they don't have a dark theme. In spite of multiple UI/ UX changes to the website, they do not stray away from their primary color theme of blue and white.
So this is always an arguable topic. But as far as facts go, applications tend to have light/ dark theme, rather than websites. And for websites like YouTube, I think they brought in the light/ dark theme, so that users can align to the themes they've set and gotten used to in their own YouTube mobile application.
But again, I could be wrong, I could be right.