Here is a solution:

Copy file vendor/easycorp/easyadmin-bundle/src/Resources/views/layout.html.twig to /templates/bundles/EasyAdminBundle/layout.html.twig and edit that file.

Comment out this line (This is on the tag somewhere around row 61):
data-ea-dark-scheme-is-enabled=”{{ ea.dashboardHasDarkModeEnabled ? ‘true’ : ‘false’ }}”

Then add ea-dark-scheme to the class=”” attribute on the body resulting in:
class=”ea {% block body_class %}{% endblock %} ea-dark-scheme”

Then copy file vendor/easycorp/easyadmin-bundle/src/Resources/views/page/login_minimal.html.twig to /templates/bundles/EasyAdminBundle/page/login_minimal.html.twig and edit that file.

Here just add ea-dark-scheme to the classes on the body like so:

To finish it off go and edit the DashboardController. Find the configureDashboard method and edit it like this:

    return Dashboard::new()
        ->setTitle('Nexus')
        ->disableDarkMode(true);

This disables the theme selector.

Doing the above results in dark theme everywhere without the theme selector visible.