{% with bg=theme.background.first %}
    {% if bg %}
        {% with css=bg.get_effective_background %}
            {% if css %}
                <style>
                :root:has(input.theme-controller[value="{{ theme.name }}"]:checked),
                [data-theme="{{ theme.name }}"] {
                    background: {{ css|safe }};
                }
                </style>
            {% endif %}
        {% endwith %}
    {% endif %}
{% endwith %}