diff --git a/nextjs-app/__tests__/components/darkThemeSafety.test.ts b/nextjs-app/__tests__/components/darkThemeSafety.test.ts
new file mode 100644
index 0000000..60445ee
--- /dev/null
+++ b/nextjs-app/__tests__/components/darkThemeSafety.test.ts
@@ -0,0 +1,78 @@
+import fs from 'fs';
+import path from 'path';
+
+/**
+ * Guards against light-theme-only CSS.
+ *
+ * The site themes entirely through tokens redefined under
+ * `@media (prefers-color-scheme: dark)`. A hardcoded colour therefore does not
+ * fail loudly — it renders perfectly in the theme it was written for and
+ * quietly wrongly in the other, which nobody sees unless they happen to be in
+ * dark mode when they look.
+ *
+ * Both rules below are drawn from real defects in SchoolHeroMap.module.css,
+ * found by eye rather than by any test:
+ *
+ *  - the map's fade to the header ramped through hardcoded white and landed on
+ *    `var(--bg-card)`. Invisible in light; a bright band across the full width
+ *    of a near-black card in dark.
+ *  - the controls floating over the map paired a hardcoded white background
+ *    with `color: var(--text-primary)`, which resolves to #E9EEF0 in dark —
+ *    near-white text on a near-white button.
+ */
+
+const COMPONENTS = path.join(__dirname, '..', '..', 'components');
+
+function stylesheets(dir: string): string[] {
+  return fs.readdirSync(dir, { withFileTypes: true }).flatMap((entry) => {
+    const full = path.join(dir, entry.name);
+    if (entry.isDirectory()) return stylesheets(full);
+    return entry.name.endsWith('.module.css') ? [full] : [];
+  });
+}
+
+/** Innermost `selector { body }` pairs. Nested at-rules never match as rules,
+ *  because their body contains braces. */
+function rules(css: string): Array<{ selector: string; body: string }> {
+  return Array.from(css.matchAll(/([^{}]+)\{([^{}]*)\}/g), (m) => ({
+    selector: m[1].trim().split('\n').pop()!.trim(),
+    body: m[2],
+  }));
+}
+
+const HARDCODED_WHITE_BG = /background[^;]*(?:255,\s*255,\s*255|#fff\b|#ffffff\b)/i;
+const THEMED_COLOR = /(?:^|[^-])color:\s*var\(--/;
+
+const files = stylesheets(COMPONENTS);
+
+describe('dark-theme safety', () => {
+  it('finds stylesheets to check', () => {
+    expect(files.length).toBeGreaterThan(0);
+  });
+
+  it('never pairs a hardcoded white background with a themed text colour', () => {
+    const offenders = files.flatMap((file) =>
+      rules(fs.readFileSync(file, 'utf8'))
+        .filter((r) => HARDCODED_WHITE_BG.test(r.body) && THEMED_COLOR.test(r.body))
+        .map((r) => `${path.relative(COMPONENTS, file)}  ${r.selector}`));
+
+    // Either the surface follows the theme and so should the text, or it does
+    // not and the text must be literal too. Mixing them is how near-white text
+    // ends up on a near-white button.
+    expect(offenders).toEqual([]);
+  });
+
+  it('never fades to a themed colour through a hardcoded one', () => {
+    const offenders = files.flatMap((file) =>
+      rules(fs.readFileSync(file, 'utf8'))
+        .filter((r) => /linear-gradient/.test(r.body)
+          && /var\(--bg-(card|primary|secondary)\)/.test(r.body)
+          && /255,\s*255,\s*255|#fff\b/i.test(r.body))
+        .map((r) => `${path.relative(COMPONENTS, file)}  ${r.selector}`));
+
+    // A gradient that lands on a token has to be made of that token, or the
+    // ramp and its destination disagree in one theme. Use the matching
+    // `--*-rgb` token for the transparent stops.
+    expect(offenders).toEqual([]);
+  });
+});
diff --git a/nextjs-app/app/globals.css b/nextjs-app/app/globals.css
index a83b438..096e4f6 100644
--- a/nextjs-app/app/globals.css
+++ b/nextjs-app/app/globals.css
@@ -28,6 +28,9 @@
     --bg-primary: #FAFAF8;      /* Warm White */
     --bg-secondary: #F5EFE6;    /* Sand — hero panels, sunken rows */
     --bg-card: #FFFFFF;
+    /* For gradients that have to fade to the card colour. A hardcoded white
+       ramp reads as a bright band against a dark card. */
+    --bg-card-rgb: 255, 255, 255;
     --surface-inverse: #0F766E;
 
     /* ── Ink ────────────────────────────────────────────────────────── */
@@ -234,6 +237,7 @@
         --bg-primary: #111A20;
         --bg-secondary: #16222A;
         --bg-card: #18242C;
+        --bg-card-rgb: 24, 36, 44;
         --surface-inverse: #E9EEF0;
 
         --text-primary: #E9EEF0;
diff --git a/nextjs-app/components/SchoolHeroMap.module.css b/nextjs-app/components/SchoolHeroMap.module.css
index 8904521..e71369c 100644
--- a/nextjs-app/components/SchoolHeroMap.module.css
+++ b/nextjs-app/components/SchoolHeroMap.module.css
@@ -47,7 +47,13 @@
     width: 100%;
     height: 100%;
     background:
-        linear-gradient(100deg, rgba(255, 255, 255, 0) 40%, rgba(255, 255, 255, .5) 50%, rgba(255, 255, 255, 0) 60%) var(--bg-secondary);
+        /* Sweeps toward the card colour, which is a shade lighter than this
+           ground in both themes. Hardcoded white was a bright flash across a
+           dark page every 1.4s while the tiles loaded. */
+        linear-gradient(100deg,
+            rgba(var(--bg-card-rgb), 0) 40%,
+            rgba(var(--bg-card-rgb), .5) 50%,
+            rgba(var(--bg-card-rgb), 0) 60%) var(--bg-secondary);
     background-size: 200% 100%;
     animation: shimmer 1.4s infinite;
 }
@@ -76,6 +82,15 @@
     justify-content: center;
 }
 
+/*
+ * Controls that float ON the map.
+ *
+ * The map tiles are light in both themes, so these deliberately do NOT follow
+ * the theme — they follow the map. The literal ink below is the point: paired
+ * with a hardcoded white background, `color: var(--text-primary)` resolved to
+ * #E9EEF0 in the dark theme and put near-white text on a near-white button.
+ * A themed token is the wrong tool for a surface that never changes.
+ */
 .openHint {
     display: inline-flex;
     align-items: center;
@@ -85,7 +100,8 @@
     border-radius: 999px;
     font-size: 13px;
     font-weight: 600;
-    color: var(--text-primary);
+    /* See "Controls that float ON the map" above. */
+    color: #1C2731;
     background: rgba(255, 255, 255, .85);
     -webkit-backdrop-filter: blur(6px);
     backdrop-filter: blur(6px);
@@ -113,11 +129,18 @@
        on top of the blend. */
     z-index: 450;
     pointer-events: none;
+    /* The card colour, not white.
+       This ramp was hardcoded white and ended at var(--bg-card). In the light
+       theme that is white into white and invisible, as intended. In the dark
+       theme it climbed to 95% WHITE and then met a near-black card — a bright
+       band across the full width, right where the map is supposed to dissolve
+       into the header. Fading to the same colour the gradient lands on is the
+       whole trick, and it only works if that colour is a token. */
     background: linear-gradient(to bottom,
-            rgba(255, 255, 255, 0) 0%,
-            rgba(255, 255, 255, .35) 35%,
-            rgba(255, 255, 255, .75) 62%,
-            rgba(255, 255, 255, .95) 82%,
+            rgba(var(--bg-card-rgb), 0) 0%,
+            rgba(var(--bg-card-rgb), .35) 35%,
+            rgba(var(--bg-card-rgb), .75) 62%,
+            rgba(var(--bg-card-rgb), .95) 82%,
             var(--bg-card) 100%);
 }
 
@@ -134,7 +157,8 @@
     border: none;
     border-radius: 8px;
     background: rgba(255, 255, 255, .92);
-    color: var(--text-primary);
+    /* See "Controls that float ON the map" above. */
+    color: #1C2731;
     cursor: pointer;
     box-shadow: 0 2px 10px rgba(var(--shadow-rgb), .2);
 }