Custom CSS Nav Menu Style Not Being Applied

Well, I found the solution. The problem was with the PHP code in the plugin itself. Here is the ‘additional_stylesheets’ function with the code that ended up working.

function additional_stylesheets() {

    // register and enqueue our style.
    wp_register_style( 'bt-wp-styles', plugins_url( 'bt-wp-styles/css/bt-wp-styles.css' ) );
    wp_enqueue_style( 'bt-wp-styles' );

}

add_action( 'wp_enqueue_scripts', 'additional_stylesheets' );

So, that’s everything. The code works great now! Thanks for looking at my answer!

Brian