Warnings and notices in debug mode

Hey Armando! I hope you are well!

I recently enabled wp-debug to see if I have any errors and I noticed some warnings and notices for FL3R Feelbox, so I post them below.

Warning: Missing argument 2 for wp_register_style(), called in /home/www/site/site.com/www/wp-content/plugins/fl3r-feelbox/feelbox.php on line 124 and defined in /home/www/site/site.com/www/wp-includes/functions.wp-styles.php on line 120

Notice: Undefined variable: src in /home/www/site/site.com/www/wp-includes/functions.wp-styles.php on line 123

Below are all the mentioned lines, I opened the files with Notepad++.

Lines 123-125 of feelbox.php:

if ( $customCSS ) {
<strong>wp_register_style('feelbox-custom-style' );</strong>
wp_enqueue_style('feelbox-custom-style');

Lines 120-123 of functions.wp-styles.php:

function wp_register_style( $handle, $src, $deps = array(), $ver = false, $media = 'all' ) {
_wp_scripts_maybe_doing_it_wrong( __FUNCTION__ );
return wp_styles()->add( $handle, $src, $deps, $ver, $media ); }

Since I’m not much familiar with coding, can you please tell me what all these mean and how can they fixed?

Please let me know.
Thanks!