How to add an alternative style sheet as a theme option?

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' ),

Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)