Remove usage of legacy AccountSettings fixture in ThemesIndexControllerTest

[cedricalfonsi]

Problem
Passbolt's migration to Fixture Factories is incomplete. Some tests still rely on legacy fixtures. The Theme capability has been partially migrated, but ThemeIndexControllerTest.php has not.

Who is impacted
Developers

Why it's important or urgent?
N/A

Solution
Update ThemeIndexControllerTest.php to use Fixture Factories instead of legacy fixtures. Other tests for the Theme capability already use Fixture Factories.

Definition of done

• ThemeIndexControllerTest.php uses Fixture Factories instead of legacy fixtures.