diff --git a/frontend/src/index.test.ts b/frontend/src/index.test.ts
new file mode 100644
index 0000000..b9d5f30
--- /dev/null
+++ b/frontend/src/index.test.ts
@@ -0,0 +1,19 @@
+import { readFileSync } from 'node:fs'
+import { resolve } from 'node:path'
+
+import { describe, expect, it } from 'vitest'
+
+
+const indexCss = readFileSync (
+    resolve (process.cwd (), 'src/index.css'),
+    'utf8')
+const compactIndexCss = indexCss.replace (/\s+/g, ' ')
+const mobileSubmenuRule = '.top-nav-mobile-menu .top-nav-submenu {'
+    + ' background: var(--top-nav-mobile-active-bg); }'
+
+
+describe ('TopNav mobile submenu colours', () => {
+  it ('uses the mobile active background for an expanded submenu', () => {
+    expect (compactIndexCss).toContain (mobileSubmenuRule)
+  })
+})