diff --git a/src/settings.js b/src/settings.js
index 92fc33c..a4684af 100644
--- a/src/settings.js
+++ b/src/settings.js
@@ -4,8 +4,22 @@ export const DEFAULT_SETTINGS = {
   recentTrails: [],
 };
 
+function deepClone(value) {
+  if (value === null || typeof value !== 'object') return value;
+  if (value instanceof Date) return new Date(value.getTime());
+  if (value instanceof Map) {
+    return new Map([...value].map(([k, v]) => [deepClone(k), deepClone(v)]));
+  }
+  if (Array.isArray(value)) return value.map(deepClone);
+  const copy = {};
+  for (const key of Object.keys(value)) {
+    copy[key] = deepClone(value[key]);
+  }
+  return copy;
+}
+
 export function applyOverrides(settings, overrides) {
-  const next = settings;
+  const next = deepClone(settings);
   for (const [key, value] of Object.entries(overrides)) {
     if (value && typeof value === 'object' && !Array.isArray(value)) {
       Object.assign(next[key], value);
diff --git a/src/settings.test.js b/src/settings.test.js
new file mode 100644
index 0000000..bcbddc1
--- /dev/null
+++ b/src/settings.test.js
@@ -0,0 +1,12 @@
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+import { DEFAULT_SETTINGS, applyOverrides } from './settings.js';
+
+test('applyOverrides leaves its input untouched', () => {
+  const before = JSON.stringify(DEFAULT_SETTINGS);
+  const next = applyOverrides(DEFAULT_SETTINGS, { units: 'imperial', theme: { mode: 'dark' } });
+  assert.equal(JSON.stringify(DEFAULT_SETTINGS), before);
+  assert.equal(next.units, 'imperial');
+  assert.deepEqual(next.theme, { mode: 'dark', accent: '#2f5d50' });
+  assert.notEqual(next.theme, DEFAULT_SETTINGS.theme);
+});