{% form_theme form '@SyliusUi/Form/theme.html.twig' %}

<div class="ui middle aligned center aligned grid">
    <div class="column">
        {{ sylius_template_event('sylius.admin.login.before_form', {'form': form}) }}

        {{ sylius_template_event('sylius.admin.login.logo', _context) }}

        {{ sylius_template_event('sylius.admin.login.error', _context) }}

        {{ form_start(form, {'action': action|default('/'), 'attr': {'class': 'ui large loadable form', 'novalidate': 'novalidate'}}) }}
            <div class="ui left aligned very padded segment">
                {{ sylius_template_event('sylius.admin.login.form.content', _context) }}
            </div>
        {{ form_end(form) }}
    </div>
</div>

{% form_theme form '@SyliusUi/Form/theme.html.twig' %}

<div class="ui middle aligned center aligned grid">
    <div class="column">
        {{ sylius_template_event('sylius.admin.login.before_form', _context) }}

        {{ form_start(form, {'action': action|default('/'), 'attr': {'class': 'ui large loadable form', 'novalidate': 'novalidate'}}) }}
            <div class="ui left aligned very padded segment">
                {{ sylius_template_event('sylius.admin.login.form.content', _context) }}
            </div>
        {{ form_end(form) }}
    </div>
</div>

Use already created event in login form
[Template][Events] Use already created event in login form
