feat(theme): persistent light/dark toggle in the titlebar

src/theme.ts

@@ -0,0 +1,16 @@
+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);
+}