When administrator add additional CSS code in theme settings, it appears as HTML entities into HTML code of the page.
1. Add some CSS (containing html special chars - quotes, <, >, etc) at Insert codes -> CSS Codes section of theme settings.
2. In source code of the page this special chars appers as HTML entities (", <, >
Filter twig variable as raw {{ code_css|raw }} in /templates/layout/html.html.twig line 50