add a test to ensure style selection works

tests/functional_tests.py

@@ -52,3 +52,24 @@ def test_that_dir_request_does_not_redirect(client):
     response = client.get('/subdir/')
     assert response.status_code == 200
     assert b'another page' in response.data
+
+
+def test_setting_selected_style_includes_cookie(client):
+    """Test that a request with style=foo sets the cookie and renders appropriately."""
+    response = client.get('/')
+    style_cookie = next((cookie for cookie in client.cookie_jar if cookie.name == 'user-style'), None)
+    assert style_cookie is None
+
+    response = client.get('/?style=light')
+    style_cookie = next((cookie for cookie in client.cookie_jar if cookie.name == 'user-style'), None)
+    assert response.status_code == 200
+    assert b'light.css' in response.data
+    assert b'dark.css' not in response.data
+    assert style_cookie.value == 'light'
+
+    response = client.get('/?style=dark')
+    style_cookie = next((cookie for cookie in client.cookie_jar if cookie.name == 'user-style'), None)
+    assert response.status_code == 200
+    assert b'dark.css' in response.data
+    assert b'light.css' not in response.data
+    assert style_cookie.value == 'dark'