It's just CSS with a little JS. Define your colours in some css variables, use the variables in your styling, then update the variables based on which selection

It's just CSS with a little JS. Define your colours in some css variables, use the variables in your styling, then update the variables based on which selection was chosen. Duckduckgo has themes and they're pretty privacy oriented. I don't think it will contribute to fingerprinting. The media prefers-color-scheme browser setting might contribute to that though, which let's you have different theme defaults depending on browser preference