From https://once.com/writebook

test/controllers/custom_styles_controller_test.rb

@@ -0,0 +1,29 @@
+require "test_helper"
+
+class Accounts::CustomStylesControllerTest < ActionDispatch::IntegrationTest
+  setup do
+    sign_in :david
+  end
+
+  test "edit" do
+    get edit_account_custom_styles_url
+    assert_response :ok
+  end
+
+  test "update" do
+    assert users(:david).administrator?
+
+    put account_custom_styles_url, params: { account: { custom_styles: ":root { --color-text: red; }" } }
+
+    assert_redirected_to edit_account_custom_styles_url
+    assert_equal accounts(:signal).custom_styles, ":root { --color-text: red; }"
+  end
+
+  test "non-admins cannot update" do
+    sign_in :kevin
+    assert users(:kevin).member?
+
+    put account_custom_styles_url, params: { account: { custom_styles: ":root { --color-text: red; }" } }
+    assert_response :forbidden
+  end
+end