{% set themeDir = '' %}
{% for site in craft.app.sites.getAllSites() %}
    {% set identities = alias('@identities') | split(',')%}
    {% for identity in identities %}
        {% set possibleSiteName = parseEnv('@' ~ identity ~ 'SiteName') %}
        {% if siteName == possibleSiteName %}
            {% set themeDir = parseEnv('@' ~ identity ~ 'ThemeDir') %}
        {% endif %}
    {% endfor %}
{% endfor %}

{% include "/" ~ themeDir ~ "/_includes/_errors/404" %}