You’re trying to execute PHP in a string – you just need:
get_template_directory_uri() . "https://wordpress.stackexchange.com/" . get_theme_mod( 'posts', 'style.css' ),
You’re trying to execute PHP in a string – you just need:
get_template_directory_uri() . "https://wordpress.stackexchange.com/" . get_theme_mod( 'posts', 'style.css' ),