diff --git a/CLAUDE.md b/CLAUDE.md
index 5d39af5..6a23635 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -37,7 +37,7 @@ E2E tests (`tests/e2e/board.spec.js`) use Playwright with Chromium. They inject
 ## Architecture
 
 The entire application lives in `index.html`:
-- **Lines 7–202**: CSS — 4 themes (`dark`, `light`, `ocean`, `amber`) defined via CSS custom properties on `:root` and `[data-theme="..."]` selectors. Theme is persisted to `localStorage` under key `mb-theme`.
+- **Lines 7–202**: CSS — 4 built-in themes (`dark`, `light`, `ocean`, `amber`) defined via CSS custom properties on `:root` and `[data-theme="..."]` selectors. Theme is persisted to `localStorage` under key `mb-theme`. A fifth `custom` theme can be loaded at runtime via the CSS plugin system (see Key Functions below).
 - **Lines 203–265**: HTML structure — `#tab-bar`, `#connect-screen`, `#board` (phases, stats, progress bar, notes), `#toast`.
 - **Lines 267–675**: Vanilla JS — no frameworks, no imports.
 
@@ -62,6 +62,9 @@ DRAG / DRAG_OVER                               // drag-and-drop transient state
 | `saveBoard(idx?)` | Writes `serialiseMD(data)` back to the file via the File System Access API |
 | `cycleFeature(pi, fi)` | Cycles a feature's status (`pending → active → done → pending`), auto-derives phase status, then saves |
 | `setTheme(t)` | Applies `data-theme` attribute and persists to `localStorage` |
+| `pickCustomTheme()` | File picker for a `.css` plugin → `loadCustomTheme` |
+| `loadCustomTheme(css)` | Injects CSS into `<style id="mb-custom-theme">`, saves to `localStorage`, activates `custom` theme |
+| `clearCustomTheme()` | Removes injected CSS, clears `localStorage`, resets to dark |
 
 Auto-save triggers on every status change and after a 1 s debounce on notes input.
 
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index be9b1b3..968155b 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -22,7 +22,7 @@ npm run test:all    # Both
 
 ## Making changes
 
-- **CSS**: Lines 7–200 of `index.html`. Themes use CSS custom properties — update all four theme blocks if adding a new variable.
+- **CSS**: Lines 7–200 of `index.html`. The four built-in themes use CSS custom properties — update all four theme blocks if adding a new variable. Custom user themes are loaded at runtime via `loadCustomTheme()` and stored in `localStorage`; see the **Custom themes** section in the README.
 - **HTML**: Lines 203–265. Keep it minimal.
 - **JS**: Lines 267–675. No imports, no modules — everything is in the global scope.
 
diff --git a/README.md b/README.md
index a9afbcc..d6b7558 100644
--- a/README.md
+++ b/README.md
@@ -81,13 +81,43 @@ The `<!-- meta: key=value ... -->` comment is optional. Recognised keys:
 - **Click to cycle** — click any feature to cycle pending → active → done
 - **Drag to reorder** — drag features within a phase, or drag phases to reorder
 - **Due dates** — colour-coded overdue/soon/future badges
-- **4 themes** — Dark, Light, Ocean, Amber (remembered across sessions)
+- **5 themes** — Dark, Light, Ocean, Amber, plus **custom CSS plugin** (load any `.css` file)
 - **Auto-save** — every status change writes back to the `.md` file immediately
 - **Notes field** — free-text area saved to the `## Notes` section
 - **Zero dependencies** — single HTML file, works offline
 
 ---
 
+## Custom themes
+
+Click **+ CSS** in the theme bar and pick any `.css` file. The file is injected as a stylesheet and remembered across sessions — no re-picking needed on reload.
+
+The plugin format is a single CSS block targeting `[data-theme="custom"]`:
+
+```css
+/* my-theme.css */
+[data-theme="custom"] {
+  --bg:          #1e1e2e;
+  --surface:     #181825;
+  --surface2:    #313244;
+  --border:      #45475a;
+  --accent:      #a6e3a1;
+  --accent-dim:  #1e3a2e;
+  --accent2:     #fab387;
+  --accent2-dim: #3d2010;
+  --blue:        #89b4fa;
+  --red:         #f38ba8;
+  --muted:       #6c7086;
+  --text:        #cdd6f4;
+  --text-dim:    #a6adc8;
+  --radius:      10px;
+}
+```
+
+Any omitted variables fall back to the dark theme. You can also include arbitrary CSS rules to customise individual components. To reset, call `clearCustomTheme()` in the browser console.
+
+---
+
 ## Requirements
 
 Chrome or Edge 86+ (uses the [File System Access API](https://developer.mozilla.org/en-US/docs/Web/API/File_System_API) to read and write local files). Firefox does not support this API.
diff --git a/index.html b/index.html
index 5236454..eb27ccb 100644
--- a/index.html
+++ b/index.html
@@ -31,6 +31,8 @@
   --text: #fef3c7; --text-dim: #d97706;
 }
 
+.theme-btn-custom { border-style: dashed; }
+
 /* ── Reset & base ───────────────────────────────────────────────────────── */
 * { box-sizing: border-box; margin: 0; padding: 0; }
 body { font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", sans-serif; background: var(--bg); color: var(--text); min-height: 100vh; }
@@ -228,6 +230,8 @@
     <button class="theme-btn"        data-theme="light" onclick="setTheme('light')">Light</button>
     <button class="theme-btn"        data-theme="ocean" onclick="setTheme('ocean')">Ocean</button>
     <button class="theme-btn"        data-theme="amber" onclick="setTheme('amber')">Amber</button>
+    <button class="theme-btn theme-btn-custom" id="custom-theme-btn" data-theme="custom" onclick="setTheme('custom')" style="display:none">Custom</button>
+    <button class="theme-btn" id="load-css-btn" onclick="pickCustomTheme()" title="Load a .css theme file">+ CSS</button>
   </div>
 </div>
 
@@ -724,6 +728,44 @@ <h3>Notes</h3>
   localStorage.setItem("mb-theme", t);
 }
 
+// ── Custom theme plugin ────────────────────────────────────────────────────
+function injectCustomCSS(cssText) {
+  let el = document.getElementById("mb-custom-theme");
+  if (!el) { el = document.createElement("style"); el.id = "mb-custom-theme"; document.head.appendChild(el); }
+  el.textContent = cssText;
+}
+
+function syncCustomBtn() {
+  const btn = document.getElementById("custom-theme-btn");
+  if (btn) btn.style.display = localStorage.getItem("mb-custom-css") ? "" : "none";
+}
+
+function loadCustomTheme(cssText) {
+  injectCustomCSS(cssText);
+  localStorage.setItem("mb-custom-css", cssText);
+  setTheme("custom");
+  syncCustomBtn();
+}
+
+async function pickCustomTheme() {
+  try {
+    const [fh] = await window.showOpenFilePicker({ types: [{ description: "CSS Theme", accept: { "text/css": [".css"] } }] });
+    const file = await fh.getFile();
+    const text = await file.text();
+    loadCustomTheme(text);
+    toast("Custom theme loaded · " + fh.name);
+  } catch(e) { if (e.name !== "AbortError") toast("Error: " + e.message); }
+}
+
+function clearCustomTheme() {
+  const el = document.getElementById("mb-custom-theme");
+  if (el) el.remove();
+  localStorage.removeItem("mb-custom-css");
+  setTheme("dark");
+  syncCustomBtn();
+  toast("Custom theme cleared");
+}
+
 // ── Utilities ──────────────────────────────────────────────────────────────
 function esc(s) { return String(s).replace(/&/g,"&amp;").replace(/</g,"&lt;").replace(/>/g,"&gt;"); }
 
@@ -741,8 +783,11 @@ <h3>Notes</h3>
 });
 
 // ── Init ───────────────────────────────────────────────────────────────────
+const savedCSS = localStorage.getItem("mb-custom-css");
+if (savedCSS) injectCustomCSS(savedCSS);
 const savedTheme = localStorage.getItem("mb-theme") || "dark";
 setTheme(savedTheme);
+syncCustomBtn();
 
 document.addEventListener("click", (e) => {
   if (!e.target.closest("#tab-add")) closeTabMenu();
@@ -751,6 +796,7 @@ <h3>Notes</h3>
 if (!window.showOpenFilePicker) {
   document.getElementById("open-btn").disabled = true;
   document.getElementById("new-btn").disabled = true;
+  document.getElementById("load-css-btn").style.display = "none";
   document.getElementById("api-hint").textContent = "⚠ Requires Chrome or Edge 86+ (File System Access API)";
   document.getElementById("api-hint").style.color = "var(--red)";
 }
diff --git a/tests/e2e/board.spec.js b/tests/e2e/board.spec.js
index 3883741..3ee545a 100644
--- a/tests/e2e/board.spec.js
+++ b/tests/e2e/board.spec.js
@@ -22,7 +22,7 @@ test.describe("MarkBoard", () => {
 
   test("theme buttons switch themes", async ({ page }) => {
     const themeButtons = page.locator(".theme-btn");
-    await expect(themeButtons).toHaveCount(4);
+    await expect(themeButtons).toHaveCount(6); // 4 built-in + Custom (hidden) + + CSS
 
     await page.locator('.theme-btn[data-theme="light"]').click();
     await expect(page.locator("html")).toHaveAttribute("data-theme", "light");