Why could I not change the icon of the default Add media button that I added into the toolbar of TinyMCE?

I used this guide to change the default Add media icon in WP:

How to replace default icon on Add Media button?

But, I had also added an Add media button into the toolbar using this codeblock:

add_filter( 'mce_buttons', 'custom_tinymce_button' );

function custom_tinymce_button( $buttons ) {

$buttons[] = 'wp_add_media';
return $buttons;
}

However, when I changed the default Add media icon, the one that appears above the editor was changed, and not the one present in the toolbar. See this:

I want to change the icon of the toolbar as well. How do I do that? Anyone got any idea?