You may edit the qa-theme.php file inside of Snowflat folder.
Search for the public function head_css()
Then, you can add extra css.
$this->output('<link rel="stylesheet" href="https://mysite.com/qa-theme/Angel/css/my_custom_style.css">');
The easiest method is to use the option in Admin > Layout - “Custom HTML in <head> section of every page”
Add your CSS between <style> and </style> - make sure to get the tags correct otherwise you may mess up the page display.
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