diff --git a/src/utils/documentExport.js b/src/utils/documentExport.js
index 90563247..2ac50847 100644
--- a/src/utils/documentExport.js
+++ b/src/utils/documentExport.js
@@ -234,7 +234,15 @@ function attachFigures(deck, files, figureIds) {
     }
     return copy;
   });
-  var out = { title: deck.title, subtitle: deck.subtitle, date: deck.date, slides: slides };
+  // theme included, and that is not incidental. Rebuilding the deck as a fresh
+  // object drops every field not named here, and theme was not named — so a
+  // deck re-skinned in the library rendered in the default palette anyway, and
+  // the picker looked broken because nothing it did ever reached the renderer.
+  // Three themes produced byte-identical files.
+  var out = {
+    title: deck.title, subtitle: deck.subtitle, date: deck.date,
+    theme: deck.theme, slides: slides
+  };
   if (deck.title && !slides.some(function (s) { return s.type === 'title'; })) {
     out.slides = [{ type: 'title', heading: deck.title, subtitle: deck.subtitle, date: deck.date }]
       .concat(slides);
diff --git a/test/deck-theme-applied.test.js b/test/deck-theme-applied.test.js
new file mode 100644
index 00000000..f444e8be
--- /dev/null
+++ b/test/deck-theme-applied.test.js
@@ -0,0 +1,47 @@
+// A deck re-skinned in the library rendered in the default palette anyway. The
+// picker saved the choice correctly and the renderer could apply it correctly;
+// the theme was dropped in between, by the step that puts the drawn figures
+// back onto the slides.
+//
+// attachFigures rebuilds the deck as a fresh object, and a fresh object keeps
+// only the fields it names. It named title, subtitle, date and slides. Three
+// different themes rendered byte-identical files.
+const test = require('node:test');
+const assert = require('node:assert/strict');
+const fs = require('node:fs');
+const path = require('node:path');
+
+const read = f => fs.readFileSync(path.join(__dirname, '..', f), 'utf8');
+
+test('the theme survives the trip to the renderer', () => {
+  const src = read('src/utils/documentExport.js');
+  const out = src.slice(src.indexOf('var out = {'), src.indexOf('var out = {') + 220);
+  assert.match(out, /theme: deck\.theme/,
+    'attachFigures drops the theme, so every export renders in the default palette');
+});
+
+test('every field the renderer reads off the deck is carried', () => {
+  // The failure was one missing key in an object literal, and it was silent.
+  // Anything the renderer reads at deck level has to be listed there.
+  const src = read('src/utils/documentExport.js');
+  const out = src.slice(src.indexOf('var out = {'), src.indexOf('var out = {') + 220);
+  for (const field of ['title', 'subtitle', 'date', 'theme', 'slides']) {
+    assert.match(out, new RegExp(field + ':'), field + ' is not carried through attachFigures');
+  }
+});
+
+test('the saved theme is what the library writes, and the renderer reads', () => {
+  // Both ends already worked; this pins that they agree on the field name.
+  const route = read('src/routes/myResources.js');
+  assert.match(route, /deck\.theme = theme/, 'the library writes deck.theme');
+  const renderer = read('scripts/render_pptx.py');
+  assert.match(renderer, /apply_theme\(/);
+  assert.match(renderer, /spec\.get\("theme"\)|deck\.get\("theme"\)|data\.get\("theme"\)/,
+    'the renderer reads the theme off the deck');
+});
+
+test('an unknown theme leaves the default rather than failing the render', () => {
+  const renderer = read('scripts/render_pptx.py');
+  const fn = renderer.slice(renderer.indexOf('def apply_theme'));
+  assert.match(fn.slice(0, 400), /a deck rendering in the wrong colours beats a deck not rendering/);
+});