diff --git a/ventures/havre/theme.js b/ventures/havre/theme.js
new file mode 100644
index 0000000..b87bcc2
--- /dev/null
+++ b/ventures/havre/theme.js
@@ -0,0 +1,19 @@
+// theme.js: light or dark, set before the page draws (so it doesn't flash), and the switch in the nav.
+// "havre.theme" is "light", "dark", or absent (follow the system). /open reads the same key.
+(function () {
+  var root = document.documentElement, K = "havre.theme";
+  function get() { try { return localStorage.getItem(K) || ""; } catch (e) { return ""; } }
+  function dark() { var t = get(); return t ? t === "dark" : !!(window.matchMedia && matchMedia("(prefers-color-scheme: dark)").matches); }
+  function apply() {
+    var t = get();
+    if (t) root.setAttribute("data-theme", t); else root.removeAttribute("data-theme");
+    var b = document.getElementById("theme");
+    if (b) { b.textContent = dark() ? "light mode" : "dark mode"; b.setAttribute("aria-pressed", dark() ? "true" : "false"); }
+  }
+  apply();
+  document.addEventListener("DOMContentLoaded", function () {
+    apply();
+    var b = document.getElementById("theme");
+    if (b) b.addEventListener("click", function () { try { localStorage.setItem(K, dark() ? "light" : "dark"); } catch (e) { /* this time only */ } apply(); });
+  });
+})();