diff --git a/webui/src/lib/theme.js b/webui/src/lib/theme.js
new file mode 100644
index 0000000..3606e6b
--- /dev/null
+++ b/webui/src/lib/theme.js
@@ -0,0 +1,16 @@
+const STORAGE_KEY = "picobot-theme";
+
+export function preferredTheme() {
+  const saved = localStorage.getItem(STORAGE_KEY);
+  if (saved === "light" || saved === "dark") return saved;
+  return matchMedia("(prefers-color-scheme: dark)").matches ? "dark" : "light";
+}
+
+export function applyTheme(theme) {
+  document.documentElement.dataset.theme = theme;
+  document.documentElement.style.colorScheme = theme;
+  document
+    .querySelector('meta[name="theme-color"]')
+    ?.setAttribute("content", theme === "dark" ? "#0b1017" : "#eef1f5");
+  localStorage.setItem(STORAGE_KEY, theme);
+}