def METHOD_NAME(
    admin_client, theme, default_theme
):
    child_theme = Theme.objects.create(name="Child Theme", parent=theme)
    edit_link = reverse("misago:admin:themes:edit", kwargs={"pk": child_theme.pk})
    admin_client.post(edit_link, {"name": child_theme.name, "parent": default_theme.pk})
    default_theme.refresh_from_db()
    child_theme.refresh_from_db()
    theme.refresh_from_db()
    assert child_theme.parent == default_theme
    assert child_theme.tree_id == default_theme.tree_id
    assert child_theme.lft == 2
    assert child_theme.rght == 3
    assert default_theme.lft == 1
    assert default_theme.rght == 4
    assert theme.tree_id != default_theme.tree_id
    assert theme.lft == 1
    assert theme.rght == 2