diff --git a/app/cypress/e2e/map-themes.cy.js b/app/cypress/e2e/map-themes.cy.js
index ddccf46f..69780566 100644
--- a/app/cypress/e2e/map-themes.cy.js
+++ b/app/cypress/e2e/map-themes.cy.js
@@ -1,3 +1,5 @@
+import {mapSprite, themeNames, themeSessionKey} from '../../src/themes/names';
+
 /*
  * The map basemap has a "flavor" of its own, and MapViewer used to hand protomaps the WROLPi
  * theme name.  That worked while the only themes were `light` and `dark`, and broke when
@@ -9,18 +11,29 @@
  * Driven through the real page in every theme, because the unit test can only check that the
  * mapping returns something valid -- it cannot see whether MapViewer USES it, which is the
  * half that was broken.
+ *
+ * `themeNames` rather than a list written out here, so a fifth theme is cold-started by this
+ * suite the day it is added rather than the day someone remembers to edit it.
  */
-const THEMES = ['light', 'dark', 'night', 'amber'];
 
 describe('the map viewer builds a basemap in every theme', () => {
-    THEMES.forEach((theme) => {
+    themeNames.forEach((theme) => {
         it(`initialises MapLibre in ${theme}`, () => {
             const errors = [];
+            /*
+             * The sprite is the half no canvas assertion can see: a wrong sprite name 404s
+             * the icons and MapLibre carries on, so the map still draws and this test would
+             * still pass.  Watching the request is the only way to tell `sprites/dark` from
+             * `sprites/black` -- and `black` is exactly the name that would be requested if
+             * the flavor and the sprite were ever collapsed back into one variable.
+             */
+            cy.intercept('GET', '**/map-assets/sprites/**').as('sprite');
+
             cy.visit('/map', {
                 onBeforeLoad(win) {
                     // Set the theme before the app boots, so the viewer builds its style once
                     // with the theme under test rather than rebuilding after a switch.
-                    win.localStorage.setItem('color-scheme', theme);
+                    win.localStorage.setItem(themeSessionKey, theme);
                     // MapViewer reports a failed init through console.error and then renders
                     // nothing, so the console is where the defect is visible.
                     const original = win.console.error;
@@ -34,6 +47,15 @@ describe('the map viewer builds a basemap in every theme', () => {
             // The canvas only exists once MapLibre constructed a style successfully.
             cy.get('canvas.maplibregl-canvas', {timeout: 40000}).should('exist');
 
+            cy.wait('@sprite', {timeout: 40000}).then(({request}) => {
+                const name = request.url
+                    .split('/map-assets/sprites/')[1]
+                    .replace(/[?#].*$/, '')
+                    .replace(/\.(json|png)$/, '')
+                    .replace('@2x', '');
+                expect(name, `sprite set requested in ${theme}`).to.equal(mapSprite(theme));
+            });
+
             cy.then(() => {
                 const failures = errors.filter(message =>
                     /MapLibre failed to initialize|reading 'background'/.test(message));
diff --git a/app/src/themes/map-flavor.test.js b/app/src/themes/map-flavor.test.js
index dc93484b..3de0ce36 100644
--- a/app/src/themes/map-flavor.test.js
+++ b/app/src/themes/map-flavor.test.js
@@ -101,3 +101,60 @@ describe('the monochrome themes get an achromatic basemap', () => {
         expect(hued.length).toBeGreaterThan(0);
     });
 });
+
+describe('every buildStyle call goes through the mapping', () => {
+    /*
+     * MapViewer builds a style in two places: once at init, and again in an effect keyed on
+     * `[theme, mapReady, scaleUnit]` that calls `map.setStyle` when any of them change.
+     *
+     * Only the first is reachable from an e2e.  The theme picker lives on Settings and in the
+     * gallery, never in the nav, so there is no way to change the theme while /map is open --
+     * navigating to Settings unmounts the viewer.  The rebuild branch is real code that a
+     * future compact picker would light up, and a regression confined to it (capturing the
+     * flavor once at init, say) would pass every browser test in this repo.
+     *
+     * So it is checked in the source.  Crude, and the honest alternative to a browser test
+     * that cannot reach the branch it claims to cover.
+     */
+    const source = fs.readFileSync(
+        path.join(__dirname, '../components/MapViewer.js'), 'utf8');
+
+    /*
+     * Paren-counting rather than a regex: the arguments are themselves calls, so anything
+     * non-greedy stops at `mapFlavor(theme)` and reports half an argument list as the whole
+     * of it -- which is how the first version of this test passed while looking wrong.
+     */
+    const buildStyleCalls = (text) => {
+        const calls = [];
+        const marker = 'buildStyle(';
+        for (let at = text.indexOf(marker); at !== -1; at = text.indexOf(marker, at + 1)) {
+            let depth = 0;
+            for (let i = at + marker.length - 1; i < text.length; i++) {
+                if (text[i] === '(') depth++;
+                else if (text[i] === ')' && --depth === 0) {
+                    calls.push(text.slice(at, i + 1));
+                    break;
+                }
+            }
+        }
+        // The `function buildStyle(...)` declaration matches too; drop it.
+        return calls.filter(call => !text.includes(`function ${call}`));
+    };
+
+    it('passes mapFlavor and mapSprite at both call sites', () => {
+        const calls = buildStyleCalls(source);
+
+        // Two call sites plus the declaration; if this drops to one the test below is
+        // covering half of what it claims.
+        expect(calls.length).toBe(2);
+        calls.forEach((call) => {
+            expect(call).toContain('mapFlavor(theme)');
+            expect(call).toContain('mapSprite(theme)');
+        });
+    });
+
+    it('never passes a bare theme as the flavor', () => {
+        // The original bug, written out: `buildStyle(sources, theme, ...)`.
+        expect(source).not.toMatch(/buildStyle\([^)]*,\s*theme\s*,/);
+    });
+});