diff --git a/CLAUDE.md b/CLAUDE.md
index afa575c..1021ccd 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -740,6 +740,21 @@ meant to stay split; add new features as new files, not inside the shells:
     as a second door. Before adding a global-looking "open X" button, check where
     X is mounted.
 
+29. **A Tailwind class whose token is undeclared fails in total silence.**
+    `frontend/tailwind.config.js` is a v3-style config and Tailwind v4 never
+    loads it — there is no `@config` in `src/index.css`. Every
+    `primary-<number>` class the app wrote therefore generated no CSS at all:
+    568 of them across 60 files, including 38 buttons carrying `text-white` on
+    a `bg-primary-600` that painted nothing, and the
+    `focus-visible:ring-primary-500` this file prescribes for accessibility.
+    No build error, no lint warning, no visual difference from a typo. The
+    scale now lives in the `@theme` block of `index.css`, which is the only
+    place v4 reads; `src/theme.test.ts` compiles the real stylesheet and fails
+    if a step stops resolving. **`--color-primary` and `--color-primary-500`
+    are different tokens** — `bg-primary` (213 uses) comes from the first and
+    must keep working. Everything else the config declares — `font-sans`,
+    `shadow-glow`, `animate-shimmer` — is still inert.
+
 28. **Two vocabularies name the same act, and they disagree on case.**
     `constants/actTypes.ts` spells it `Regolamento UE`; the backend resolver
     answers `regolamento ue`. A `===` between the two silently produced an act
diff --git a/docs/deployment.md b/docs/deployment.md
index 749ba91..bbe4f9a 100644
--- a/docs/deployment.md
+++ b/docs/deployment.md
@@ -95,7 +95,7 @@ cd backend && npm test
 cd frontend && npm run build && npx vitest run
 ```
 
-Expected today: 400 Python (1 deselected — the `live` marker), 35 backend, 192 frontend.
+Expected today: 400 Python (1 deselected — the `live` marker), 35 backend, 194 frontend.
 
 The backend suite needs `backend/.env.test` pointing at a **separate** database
 (`visualex_test`, not `visualex_platform`) — it runs `prisma migrate reset` on
diff --git a/frontend/src/index.css b/frontend/src/index.css
index 2c8719c..e786585 100644
--- a/frontend/src/index.css
+++ b/frontend/src/index.css
@@ -43,6 +43,25 @@
   --color-warning: hsl(var(--warning));
   --color-warning-foreground: hsl(var(--warning-foreground));
 
+  /* Primary scale — Blue.
+     These lived in tailwind.config.js, which is a v3-style config: Tailwind v4
+     never loads it without an `@config` directive, so every `primary-<number>`
+     class in the app generated nothing at all. Declared here instead, where v4
+     actually reads them. Values are Tailwind's own blue, unchanged from the
+     config. `--color-primary` above is a different token and stays as it is:
+     `bg-primary` and `primary-500` are not the same thing. */
+  --color-primary-50: #eff6ff;
+  --color-primary-100: #dbeafe;
+  --color-primary-200: #bfdbfe;
+  --color-primary-300: #93c5fd;
+  --color-primary-400: #60a5fa;
+  --color-primary-500: #3b82f6;
+  --color-primary-600: #2563eb;
+  --color-primary-700: #1d4ed8;
+  --color-primary-800: #1e40af;
+  --color-primary-900: #1e3a8a;
+  --color-primary-950: #172554;
+
   /* Radius */
   --radius-lg: var(--radius);
   --radius-md: calc(var(--radius) - 2px);
diff --git a/frontend/src/theme.test.ts b/frontend/src/theme.test.ts
new file mode 100644
index 0000000..ccce61f
--- /dev/null
+++ b/frontend/src/theme.test.ts
@@ -0,0 +1,41 @@
+import { describe, it, expect } from 'vitest';
+import { readFileSync } from 'node:fs';
+import { resolve } from 'node:path';
+import postcss from 'postcss';
+import tailwind from '@tailwindcss/postcss';
+
+/**
+ * Tailwind failures are silent: a class whose token the theme never declares
+ * generates no CSS, and nothing — not the build, not the linter — says so. That
+ * is how `primary-<number>` came to be written 568 times while drawing nothing
+ * (tailwind.config.js is a v3-style config, and v4 never loads it without an
+ * `@config` directive). These tests compile the real stylesheet and assert the
+ * tokens the app spends its classes on actually resolve.
+ */
+// vitest runs with the frontend package as its root.
+const CSS_PATH = resolve(process.cwd(), 'src/index.css');
+
+async function compile(candidates: string[]): Promise<string> {
+  const source = `${readFileSync(CSS_PATH, 'utf8')}\n@source inline("${candidates.join(' ')}");\n`;
+  const { css } = await postcss([tailwind()]).process(source, { from: CSS_PATH });
+  return css;
+}
+
+describe('the primary colour scale', () => {
+  it('gives every step used by the app a real colour', async () => {
+    const steps = [50, 100, 200, 300, 400, 500, 600, 700, 800, 900, 950];
+    const css = await compile(steps.map((s) => `bg-primary-${s}`));
+
+    for (const step of steps) {
+      expect(css, `bg-primary-${step} generated no rule`).toContain(`.bg-primary-${step}`);
+    }
+  }, 30000);
+
+  it('still resolves the unnumbered primary token, which 213 classes rely on', async () => {
+    const css = await compile(['bg-primary', 'ring-ring', 'bg-background']);
+
+    expect(css).toContain('.bg-primary');
+    expect(css).toContain('.ring-ring');
+    expect(css).toContain('.bg-background');
+  }, 30000);
+});
diff --git a/frontend/tailwind.config.js b/frontend/tailwind.config.js
index ddacc3d..2cadd08 100644
--- a/frontend/tailwind.config.js
+++ b/frontend/tailwind.config.js
@@ -1,3 +1,16 @@
+/**
+ * NOT LOADED. This is a Tailwind v3-style config; the project runs Tailwind v4
+ * (`@import "tailwindcss"` + `@theme` in src/index.css) and v4 only reads a JS
+ * config when the stylesheet asks for it with `@config`. Nothing here reaches
+ * the build: editing a value in this file changes nothing you can see.
+ *
+ * The `primary` scale below is live only because it was copied into the
+ * `@theme` block in src/index.css — edit it there. `fontFamily`, `boxShadow`
+ * (`shadow-glow`, `shadow-glass-lg`) and the keyframes/animations are still
+ * inert, and the fonts named here are never fetched anyway.
+ *
+ * Kept for the record until we decide whether to wire it up or delete it.
+ */
 /** @type {import('tailwindcss').Config} */
 export default {
   content: [