Why don't you just declare html.light and html.dark and use js just to switch this classes?

setTheme(theme){
    if (theme == "dark") {
        document.querySelector("html").classList.remove("light");
        document.querySelector("html").classList.add("dark");
    } else {       
        document.querySelector("html").classList.remove("dark");
        document.querySelector("html").classList.add("light");
    }
}