margin/src/theme.ts

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

const KEY = "margin-theme";

export function initialTheme(): Theme {
  const attr = document.documentElement.getAttribute("data-theme");
  if (attr === "light" || attr === "dark") return attr;
  const saved = localStorage.getItem(KEY);
  if (saved === "light" || saved === "dark") return saved;
  return window.matchMedia("(prefers-color-scheme: dark)").matches ? "dark" : "light";
}

export function applyTheme(theme: Theme) {
  document.documentElement.setAttribute("data-theme", theme);
  localStorage.setItem(KEY, theme);
}