diff --git a/apps/desktop/e2e/appearance.spec.ts b/apps/desktop/e2e/appearance.spec.ts
index ec9922c..7ed7201 100644
--- a/apps/desktop/e2e/appearance.spec.ts
+++ b/apps/desktop/e2e/appearance.spec.ts
@@ -11,9 +11,14 @@ import {
 } from "@playwright/test";
 import { autoSaveDialog, downloadPdf, maximizeWindow } from "./helpers";
 
-/** Light and dark, in Notion's black and white, set in monospace: what is on screen is read from the page's own styles. */
+/** Neutral light and dark surfaces with semantic accents, set in monospace: values are read from the page's own styles. */
 const LIGHT = { bg: "rgb(255, 255, 255)", ink: "rgb(44, 44, 43)" };
-const DARK = { bg: "rgb(25, 25, 25)", ink: "rgb(240, 239, 237)" };
+const DARK = {
+  bg: "rgb(25, 25, 25)",
+  ink: "rgb(240, 239, 237)",
+  structure: "rgb(140, 180, 255)",
+  structureInk: "rgb(25, 25, 25)",
+};
 
 async function launch(): Promise<{ app: ElectronApplication; page: Page }> {
   const home = mkdtempSync(join(tmpdir(), "lucid-home-"));
@@ -58,10 +63,10 @@ test("it is light by default, the sidebar's moon switches to dark, and the choic
     expect(await theme(page)).toBe("dark");
     expect(await style(page, "body", "background-color")).toBe(DARK.bg);
     expect(await style(page, "body", "color")).toBe(DARK.ink);
-    // The one filled button is the text colour, inverted, so it stays black on white and white on black.
+    // Primary actions use the structure signal, with the neutral canvas preserved around them.
     const download = page.getByRole("button", { name: "Download PDF" });
-    await expect(download).toHaveCSS("background-color", DARK.ink);
-    await expect(download).toHaveCSS("color", DARK.bg);
+    await expect(download).toHaveCSS("background-color", DARK.structure);
+    await expect(download).toHaveCSS("color", DARK.structureInk);
     await expect(page.getByRole("button", { name: "Switch to light mode" })).toBeVisible();
     await page.screenshot({ path: join(__dirname, "../test-results/appearance-dark.png") });
 
diff --git a/apps/desktop/electron/contrast.test.ts b/apps/desktop/electron/contrast.test.ts
index b14f85a..78bab3c 100644
--- a/apps/desktop/electron/contrast.test.ts
+++ b/apps/desktop/electron/contrast.test.ts
@@ -38,4 +38,16 @@ describe.each([
     expect(ratio(get("--ink-2"), get("--bg"))).toBeGreaterThanOrEqual(4.5);
     expect(ratio(get("--ink-3"), get("--bg"))).toBeGreaterThanOrEqual(3);
   });
+
+  it("keeps semantic signals readable without relying on colour alone", () => {
+    for (const name of ["structure", "data", "decision", "attention"]) {
+      expect(ratio(get(`--${name}`), get(`--${name}-bg`)), name).toBeGreaterThanOrEqual(4.5);
+    }
+    for (const name of ["structure", "data", "decision"]) {
+      expect(
+        ratio(get(`--${name}-ink`), get(`--${name}`)),
+        `${name} button`,
+      ).toBeGreaterThanOrEqual(4.5);
+    }
+  });
 });
diff --git a/apps/desktop/electron/site.test.ts b/apps/desktop/electron/site.test.ts
index 439c537..ad02b00 100644
--- a/apps/desktop/electron/site.test.ts
+++ b/apps/desktop/electron/site.test.ts
@@ -180,6 +180,8 @@ describe.each([
     expect(contrastRatio(get("--structure"), get("--structure-bg"))).toBeGreaterThanOrEqual(4.5);
     expect(contrastRatio(get("--data"), get("--data-bg"))).toBeGreaterThanOrEqual(4.5);
     expect(contrastRatio(get("--decision"), get("--decision-bg"))).toBeGreaterThanOrEqual(4.5);
+    expect(contrastRatio(get("--attention"), get("--attention-bg"))).toBeGreaterThanOrEqual(4.5);
+    expect(contrastRatio(get("--structure-ink"), get("--structure"))).toBeGreaterThanOrEqual(4.5);
   });
 });
 
diff --git a/apps/desktop/src/styles.css b/apps/desktop/src/styles.css
index 089c070..2efa1ca 100644
--- a/apps/desktop/src/styles.css
+++ b/apps/desktop/src/styles.css
@@ -1,10 +1,10 @@
 /*
- * Lucid's look: Notion's design language in black and white, in a light and a dark theme, set in monospace by default.
+ * Lucid's look: a neutral writing surface with a small semantic palette, in light and dark, set in monospace by default.
  *
  * The colours are Notion's own neutral ramp (text #2c2c2b on white, #f0efed on #191919 in the dark theme; warm greys for
  * lines, hovers and panels). Surfaces are flat and separated by hairlines, buttons are quiet (6 px corners, no gloss),
- * and the one filled button per view is black on light and white on dark. The only other colours are the red of an
- * error and the small status dots.
+ * and the one filled button per view carries the engineering-blue structure colour. Violet identifies commands and AI
+ * context, green decisions and success, amber attention, and red remains reserved for errors and destructive actions.
  *
  * Everything below reads the tokens in the first two blocks, so a theme is one block of variables.
  */
@@ -35,6 +35,21 @@
   --red-bg: rgb(207 81 72 / 9%);
   --ok: #4b9a78;
   --warn: #c99a2e;
+  --structure: #315eba;
+  --structure-bg: #e8f0ff;
+  --structure-line: #adc2ed;
+  --structure-ink: #ffffff;
+  --data: #6d4e9c;
+  --data-bg: #f1ebf8;
+  --data-line: #cdbde2;
+  --data-ink: #ffffff;
+  --decision: #2f7657;
+  --decision-bg: #e8f3ed;
+  --decision-line: #b2d5c3;
+  --decision-ink: #ffffff;
+  --attention: #8a5a00;
+  --attention-bg: #fff3d6;
+  --attention-line: #dfc17c;
   --shadow-sm: 0 1px 2px rgb(15 15 15 / 8%);
   --shadow-box: 0 1px 2px rgb(15 15 15 / 6%), 0 6px 20px rgb(15 15 15 / 7%);
   --shadow-pop:
@@ -83,6 +98,21 @@
   --red-bg: rgb(229 100 88 / 12%);
   --ok: #5fb892;
   --warn: #d9b04a;
+  --structure: #8cb4ff;
+  --structure-bg: #202f49;
+  --structure-line: #435d86;
+  --structure-ink: #191919;
+  --data: #c2a7e6;
+  --data-bg: #322940;
+  --data-line: #5d4b72;
+  --data-ink: #191919;
+  --decision: #75c7a4;
+  --decision-bg: #20352c;
+  --decision-line: #3d6957;
+  --decision-ink: #191919;
+  --attention: #f2c565;
+  --attention-bg: #3d3017;
+  --attention-line: #705824;
   --shadow-sm: 0 1px 2px rgb(0 0 0 / 40%);
   --shadow-box: 0 1px 2px rgb(0 0 0 / 30%), 0 6px 24px rgb(0 0 0 / 35%);
   --shadow-pop:
@@ -2797,3 +2827,182 @@ select:focus-visible,
     background: transparent;
   }
 }
+
+/* ---- semantic colour layer -------------------------------------------------
+ * This is intentionally isolated from the neutral layout rules above. Removing this block and the semantic tokens in the
+ * two theme declarations restores Lucid's earlier monochrome UI without changing markup, settings, or document data.
+ */
+.primary {
+  border-color: var(--structure);
+  background: var(--structure);
+  color: var(--structure-ink);
+}
+
+.page-row[data-active="true"] {
+  border-color: var(--structure-line);
+  box-shadow:
+    var(--shadow-sm),
+    inset 3px 0 0 var(--structure);
+}
+.page-row[data-active="true"] .file-body {
+  fill: var(--structure-bg);
+  stroke: var(--structure);
+}
+.page-row[data-active="true"] .file-fold,
+.page-row[data-active="true"] .file-lines,
+.block-no[aria-expanded="true"] {
+  stroke: var(--structure);
+  color: var(--structure);
+}
+
+.dictation[data-mode="input"] .dictation-box {
+  border-color: var(--structure-line);
+  box-shadow:
+    var(--shadow-box),
+    inset 3px 0 0 var(--structure);
+}
+.dictation[data-mode="command"] .dictation-box {
+  border-color: var(--data-line);
+  box-shadow:
+    var(--shadow-box),
+    inset 3px 0 0 var(--data);
+}
+.dictation[data-mode="input"] .dictation-box:focus-within {
+  box-shadow:
+    0 0 0 2px var(--structure-bg),
+    var(--shadow-box),
+    inset 3px 0 0 var(--structure);
+}
+.dictation[data-mode="command"] .dictation-box:focus-within {
+  box-shadow:
+    0 0 0 2px var(--data-bg),
+    var(--shadow-box),
+    inset 3px 0 0 var(--data);
+}
+.dictation[data-mode="input"] .send:not(:disabled) {
+  background: var(--structure);
+  color: var(--structure-ink);
+}
+.dictation[data-mode="command"] .send:not(:disabled) {
+  background: var(--data);
+  color: var(--data-ink);
+}
+.dictation[data-mode="input"] .dots i {
+  background: var(--structure);
+}
+.dictation[data-mode="command"] .dots i {
+  background: var(--data);
+}
+.dictation[data-status="done"] .dictation-line .tick {
+  color: var(--decision);
+}
+
+.mode-option[data-mode="input"][aria-pressed="true"] {
+  background: var(--structure-bg);
+  color: var(--structure);
+  box-shadow: inset 0 0 0 1px var(--structure-line);
+}
+.mode-option[data-mode="command"][aria-pressed="true"] {
+  background: var(--data-bg);
+  color: var(--data);
+  box-shadow: inset 0 0 0 1px var(--data-line);
+}
+.mode-option.build[aria-pressed="true"],
+.wb-tabs button[aria-selected="true"] {
+  border-color: var(--structure);
+  background: var(--structure-bg);
+  color: var(--structure);
+  box-shadow: inset 0 0 0 1px var(--structure-line);
+}
+
+.dictation-card[data-card="proposal"] {
+  border-color: var(--data-line);
+  background: var(--data-bg);
+}
+.dictation-card[data-card="clarify"] {
+  border-color: var(--attention-line);
+  background: var(--attention-bg);
+  box-shadow: inset 3px 0 0 var(--attention);
+}
+.dictation-card[data-card="content"] {
+  border-color: var(--structure-line);
+  background: var(--structure-bg);
+}
+
+.callout[data-kind="decision"] .callout-kind,
+.bn-block-content[data-text-color="green"] {
+  color: var(--decision);
+}
+.bn-block-outer:has(.callout[data-kind="decision"]) {
+  border-color: var(--decision-line);
+  background: var(--decision-bg);
+}
+.callout[data-kind="question"] .callout-kind,
+.callout[data-kind="tradeoff"] .callout-kind {
+  color: var(--data);
+}
+.bn-block-outer:has(.callout[data-kind="question"]),
+.bn-block-outer:has(.callout[data-kind="tradeoff"]) {
+  border-color: var(--data-line);
+  background: var(--data-bg);
+}
+.bn-block-outer:has(.callout[data-kind="warning"]) {
+  border-color: var(--attention-line);
+  background: var(--attention-bg);
+}
+.bn-block-outer:has(.callout[data-kind="warning"]) .callout-kind {
+  color: var(--attention);
+}
+.callout[data-status="chosen"],
+.math-block[data-status="chosen"],
+.figure[data-status="chosen"] {
+  outline-color: var(--decision);
+}
+
+.wb-legend li[data-fresh="true"],
+.wb-grid [data-fresh="true"] {
+  background: var(--structure-bg);
+}
+.wb-grid td[data-cursor="true"] {
+  outline-color: var(--structure);
+  background: var(--structure-bg);
+}
+.np-format[data-selected="true"],
+.np-blank[data-selected="true"] {
+  border-color: var(--structure);
+  background: var(--structure-bg);
+  box-shadow: 0 0 0 1px var(--structure);
+}
+.provider-list > button[aria-current="true"] {
+  border-color: var(--structure-line);
+  background: var(--structure-bg);
+  color: var(--structure);
+}
+.provider-result[data-kind="success"] {
+  border-left-color: var(--decision);
+  background: var(--decision-bg);
+}
+
+.history-mode[data-mode="agent"],
+.history-mode[data-mode="change"] {
+  color: var(--data);
+}
+.side li[data-source="agent"] {
+  border-left-color: var(--data);
+}
+.page-editor .bn-block-content.ProseMirror-selectednode > :after,
+.page-editor .ProseMirror-selectednode > .bn-block-content > :after,
+.page-editor .bn-block-content .ProseMirror-selectednode:after,
+.page-editor .bn-inline-content .ProseMirror-selectednode:after {
+  background-color: color-mix(in srgb, var(--structure-bg) 72%, transparent);
+  box-shadow: inset 0 0 0 2px var(--structure);
+}
+@keyframes flash {
+  0%,
+  30% {
+    background: var(--structure-bg);
+  }
+  100% {
+    background: transparent;
+  }
+}
diff --git a/site/style.css b/site/style.css
index 54fcfd9..df6ad68 100644
--- a/site/style.css
+++ b/site/style.css
@@ -39,10 +39,17 @@
   --red: #b8453d;
   --structure: #315eba;
   --structure-bg: #e8f0ff;
+  --structure-line: #adc2ed;
+  --structure-ink: #ffffff;
   --data: #6d4e9c;
   --data-bg: #f1ebf8;
+  --data-line: #cdbde2;
   --decision: #2f7657;
   --decision-bg: #e8f3ed;
+  --decision-line: #b2d5c3;
+  --attention: #8a5a00;
+  --attention-bg: #fff3d6;
+  --attention-line: #dfc17c;
   --shadow-1: 0 1px 2px rgb(15 15 15 / 6%), 0 6px 20px rgb(15 15 15 / 6%);
   --shadow-window: 0 2px 4px rgb(15 15 15 / 6%), 0 24px 64px -12px rgb(15 15 15 / 22%);
 
@@ -88,10 +95,17 @@
   --red: #e56458;
   --structure: #8cb4ff;
   --structure-bg: #202f49;
+  --structure-line: #435d86;
+  --structure-ink: #191919;
   --data: #c2a7e6;
   --data-bg: #322940;
+  --data-line: #5d4b72;
   --decision: #75c7a4;
   --decision-bg: #20352c;
+  --decision-line: #3d6957;
+  --attention: #f2c565;
+  --attention-bg: #3d3017;
+  --attention-line: #705824;
   --shadow-1: 0 1px 2px rgb(0 0 0 / 40%), 0 6px 24px rgb(0 0 0 / 35%);
   --shadow-window: 0 2px 4px rgb(0 0 0 / 40%), 0 24px 64px -12px rgb(0 0 0 / 70%);
 }
@@ -112,10 +126,17 @@
     --red: #e56458;
     --structure: #8cb4ff;
     --structure-bg: #202f49;
+    --structure-line: #435d86;
+    --structure-ink: #191919;
     --data: #c2a7e6;
     --data-bg: #322940;
+    --data-line: #5d4b72;
     --decision: #75c7a4;
     --decision-bg: #20352c;
+    --decision-line: #3d6957;
+    --attention: #f2c565;
+    --attention-bg: #3d3017;
+    --attention-line: #705824;
     --shadow-1: 0 1px 2px rgb(0 0 0 / 40%), 0 6px 24px rgb(0 0 0 / 35%);
     --shadow-window: 0 2px 4px rgb(0 0 0 / 40%), 0 24px 64px -12px rgb(0 0 0 / 70%);
   }
@@ -840,6 +861,58 @@ h3 {
   }
 }
 
+/* ---- semantic colour layer -------------------------------------------------
+ * The website and app share these meanings. This block is deliberately separate from the neutral layout so the broader
+ * palette can be reverted without disturbing typography, spacing, or the thought-to-structure diagram above.
+ */
+a {
+  text-decoration-color: var(--structure-line);
+}
+a:hover,
+.nav a:hover {
+  text-decoration-color: var(--structure);
+  color: var(--structure);
+}
+:focus-visible {
+  outline-color: var(--structure);
+}
+.btn.primary,
+.tabs button[aria-selected="true"] {
+  border-color: var(--structure);
+  background: var(--structure);
+  color: var(--structure-ink);
+}
+.hero::before {
+  background-image: radial-gradient(var(--structure-line) 1px, transparent 1px);
+}
+.eyebrow {
+  border-color: var(--structure-line);
+  background: var(--structure-bg);
+}
+.eyebrow i {
+  background: var(--structure);
+}
+.steps li {
+  border-top-color: var(--structure);
+}
+.steps li::before {
+  color: var(--structure);
+}
+.eval-decision {
+  border-color: var(--decision-line);
+  box-shadow: inset 3px 0 0 var(--decision);
+  padding-inline: var(--s5);
+  background: var(--decision-bg);
+}
+.eval-decision strong,
+.eval-table tbody tr:first-child .eval-verdict {
+  color: var(--decision);
+}
+#agents .code {
+  border-color: var(--data-line);
+  box-shadow: inset 3px 0 0 var(--data);
+}
+
 /* ---- how it works ---- */
 .steps {
   display: grid;