Why is the “Additional CSS” section missing in my theme Customizer?

I finally found the answer myself after a year and a half.

I’m not sure how this value was changed but changing:

define('DISALLOW_UNFILTERED_HTML', true);

in wp-config.php back to:

define('DISALLOW_UNFILTERED_HTML', false);

has fixed the problem.

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