diff --git a/docs/development.adoc b/docs/development.adoc
index 596f28f..7d49a2a 100644
--- a/docs/development.adoc
+++ b/docs/development.adoc
@@ -23,6 +23,17 @@ explicit cache headers, so an edit to `public/` (or `views/`) can
 appear not to have taken effect on the next reload. Force a real
 reload (e.g. Ctrl/Cmd+Shift+R) if that happens.
 
+`/theme_preview.html` is a standalone debug page (not linked from the
+app itself) for comparing offline base-map themes side by side: a bare
+`protomaps-themes-base` theme, an iframe of the real app's offline
+style, and one of its online style, with pan/zoom kept in sync across
+all three via the same URL-hash mechanism `map.js` already uses for
+shareable links. It's a plain static file, so nothing gates it to
+development — it's reachable on a production instance too, same as any
+other file under `public/`. Harmless if it stays that way (it shows
+nothing `/` doesn't already), but worth knowing if that's ever not the
+case.
+
 == Offline map data
 
 To enable the offline style, place a PMTiles file at
diff --git a/public/theme_preview.html b/public/theme_preview.html
new file mode 100644
index 0000000..c5eac67
--- /dev/null
+++ b/public/theme_preview.html
@@ -0,0 +1,111 @@
+<!DOCTYPE html>
+<html lang="en">
+<head>
+<meta charset="utf-8">
+<title>Offline theme preview</title>
+<link rel="stylesheet" href="/vendor/maplibre-gl/maplibre-gl.css">
+<style>
+  body { margin: 0; font-family: sans-serif; background: #222; }
+  .grid { display: grid; grid-template-columns: repeat(3, 1fr); gap: 4px; }
+  .cell { position: relative; height: 90vh; }
+  .cell .map, .cell iframe { position: absolute; inset: 0; width: 100%; height: 100%; border: 0; }
+  .cell .label {
+    position: absolute; top: 4px; left: 4px; z-index: 1;
+    background: rgba(0,0,0,.6); color: #fff; padding: 2px 8px;
+    font-size: 13px; border-radius: 3px;
+  }
+</style>
+</head>
+<body>
+<div class="grid">
+  <div class="cell">
+    <div class="label">light (bare base theme)</div>
+    <div class="map" id="light-map"></div>
+  </div>
+  <div class="cell">
+    <div class="label">offline (current app)</div>
+    <iframe id="offline-frame" src="/"></iframe>
+  </div>
+  <div class="cell">
+    <div class="label">online (current app)</div>
+    <iframe id="online-frame" src="/"></iframe>
+  </div>
+</div>
+<script src="/vendor/pmtiles/pmtiles.js"></script>
+<script src="/vendor/protomaps-themes-base/protomaps-themes-base.js"></script>
+<script type="module">
+  import { Map as MaplibreMap, addProtocol } from "/vendor/maplibre-gl/maplibre-gl.mjs";
+
+  const protocol = new pmtiles.Protocol();
+  addProtocol("pmtiles", protocol.tile);
+
+  const CENTER = [17.1077, 48.1486];
+  const ZOOM = 13;
+  const pmtilesUrl = `${window.location.origin}/tiles/offline.pmtiles`;
+
+  const lightMap = new MaplibreMap({
+    container: "light-map",
+    center: CENTER,
+    zoom: ZOOM,
+    style: {
+      version: 8,
+      glyphs: `${window.location.origin}/vendor/fonts/{fontstack}/{range}.pbf`,
+      sources: {
+        offline: { type: "vector", url: `pmtiles://${pmtilesUrl}` },
+      },
+      layers: protomaps_themes_base.layers("offline", protomaps_themes_base.namedTheme("light"), { lang: "en" }),
+    },
+  });
+
+  // "offline" frame needs nothing: the app already defaults to its offline
+  // style whenever /api/offline-tiles reports data present (see map.js).
+  // "online" frame: click its own real Online button, same button a user
+  // would click — reuses the app's actual ONLINE_STYLE (patched Liberty)
+  // instead of re-fetching/re-patching a second copy here.
+  const offlineFrame = document.getElementById("offline-frame");
+  const onlineFrame = document.getElementById("online-frame");
+  const clickOnline = () => {
+    const button = onlineFrame.contentDocument?.querySelector('.basemap-panel__option[data-style="online"]');
+    if (button) button.click();
+  };
+  onlineFrame.addEventListener("load", () => {
+    clickOnline();
+    setTimeout(clickOnline, 1000); // retry once in case map.js was still mid-init
+  });
+
+  // Keep all three views looking at the same spot. The two iframes are the
+  // real app (separate documents, own module scope — nothing to reach into
+  // directly), but map.js already syncs its map to the URL hash (`hash:
+  // "map"`, format #map=zoom/lat/lon) for shareable links. Reusing that: poll
+  // each source's position, and whichever moved gets pushed to the other
+  // two — for a frame, by writing its location.hash, which fires its own
+  // hashchange and lets its Hash control jumpTo for us.
+  const HASH_RE = /map=([\d.]+)\/(-?[\d.]+)\/(-?[\d.]+)/;
+  function frameHash(frame) {
+    const match = HASH_RE.exec(frame.contentWindow?.location.hash ?? "");
+    return match ? { zoom: +match[1], lat: +match[2], lon: +match[3] } : null;
+  }
+  function lightPos() {
+    const c = lightMap.getCenter();
+    return { zoom: lightMap.getZoom(), lat: c.lat, lon: c.lng };
+  }
+  function samePos(a, b) {
+    return Math.abs(a.zoom - b.zoom) < 0.005 && Math.abs(a.lat - b.lat) < 1e-4 && Math.abs(a.lon - b.lon) < 1e-4;
+  }
+  let current = { zoom: ZOOM, lat: CENTER[1], lon: CENTER[0] };
+  setInterval(() => {
+    const sources = { light: lightPos(), offline: frameHash(offlineFrame), online: frameHash(onlineFrame) };
+    const leader = Object.keys(sources).find((name) => sources[name] && !samePos(sources[name], current));
+    if (!leader) return;
+    current = sources[leader];
+    if (leader !== "light") lightMap.jumpTo({ center: [current.lon, current.lat], zoom: current.zoom });
+    if (leader !== "offline" && offlineFrame.contentWindow) {
+      offlineFrame.contentWindow.location.hash = `map=${current.zoom}/${current.lat}/${current.lon}`;
+    }
+    if (leader !== "online" && onlineFrame.contentWindow) {
+      onlineFrame.contentWindow.location.hash = `map=${current.zoom}/${current.lat}/${current.lon}`;
+    }
+  }, 150);
+</script>
+</body>
+</html>