functions.php in lifter launchpad theme

Hi,
I have installed lifterlms launchpad, but I have some problems about functions.php in the lifter launchpad theme.

The functions I created could not work as expected.

For example:
1. I created my own wp_head function as below to load css, but it didn’t load the functions.
function add_css_head() {
if (!is_user_logged_in()) { //if not logged in
?>
<style>
.btn-forside {
display: none !important;
}
</style>

<?php

} else { //if logged in
?>
<style>
.reg-forside {
display: none !important;
}
</style>

<?php
}
}
add_action( ‘wp_head’, ‘add_css_head’ ,10);

2. The page turned out to be blank, because “the_content” function didn’t return the content I added as below:
add_filter(‘the_content’, ‘add_content’);
function test_add_content($content)
{
return $content.”test add content”;
}`

The page I need help with: [log in to see the link]