Load a custom stylesheet programmatically

add_action( 'wp_enqueue_scripts', function() {
    
    // Note: this just dequeues the wp-members default.
    // This can be used just to load a custom stylesheet as well,
    // in which case you can omit this particlar call.
    wp_dequeue_style( 'wp-members' );
    
    // Give your style a "handle" and identify the URL path to the stylesheet.
    // Note: test the URL in your browser. If it doesn't load in your browser
    // it won't load in the page either.
    wp_enqueue_style( 'my-style', 'https://mysite.com/my/path/to/custom.css' );
    
},100);