Normally I would agree but as a maintainer you want your readme to be readable for everyone (or at least the most amount of people possible). In general, your project should be acessible and endure a wide range of system configurations, environments and opinions.
For example, I personally don’t understand why people use yarn or a four years old node.js version or Angular, and we could argue about it for hours, but instead I’ll just make sure these people can use my project as it is, no matter how dumb their configuration/opinion is in my mind. Ideally. These are popular and established, it would just be stupid to make my project somehow “incompatible” with them, if I want people to interact with it.
Sure, I can turn off dark mode but I’m not writing my readmes for myself, but for everyone else, of which apparently a lot enabled dark mode already.