… officially a term of art in computer science.
Because it’s a part of Chrome Platform Status as a feature of Client Hints. Indeed,
prefers-color-scheme is a perfect thing for Client Hints.
… it is a best practice to not load CSS for the particular non-matching color scheme in the critical rendering path, and instead to initially only load the currently relevant CSS.
No FART if the server only returns CSS matching your preferred theme (right away, anyway). Client Hints is the mechanism that tells the server that preference so it can respond accordingly.