Pretend another theme is active in the WP dashboard to maintain access to everything when something in your theme messes everything up (or whatevs).

mu-plugin-file.php

<?php

add_filter( 'template', 'onx_change_theme' );
add_filter( 'option_template', 'onx_change_theme' );
add_filter( 'option_stylesheet', 'onx_change_theme' );
add_filter( 'pre_option_stylesheet', 'onx_change_theme' );
function onx_change_theme( $theme )
{
    if ( is_admin() && ( function_exists( 'wp_get_current_user' ) ) )
    {
    	if ( current_user_can( 'administrator' ) )
        	return 'twentynineteen';
    }

    return $theme;
}