export type Theme = "light" | "dark" | "system";

function storage(): Storage | null {
  if (typeof globalThis === "undefined" || !("localStorage" in globalThis)) return null;
  const candidate = globalThis.localStorage;
  if (!candidate || typeof candidate.getItem !== "function" || typeof candidate.setItem !== "function") return null;
  return candidate;
}

export function getTheme(): Theme {
  return (storage()?.getItem("theme") as Theme) || "system";
}

export function setTheme(theme: Theme) {
  storage()?.setItem("theme", theme);
  applyTheme(theme);
}

export function applyTheme(theme: Theme) {
  const prefersDark =
    typeof globalThis !== "undefined" && typeof globalThis.matchMedia === "function" && globalThis.matchMedia("(prefers-color-scheme: dark)").matches;
  const isDark = theme === "dark" || (theme === "system" && prefersDark);

  document.documentElement.classList.toggle("dark", isDark);
}