for me this is way easier: insert the following line into the custom CSS field:
@import url('/wp-content/themes/mylay/additionalstyles.css');
Then you can save a css file named additionalstyles.css in the themes-folder /mylay/ā¦, and this file you can open with for example Transmit (FTP-Client) & Sublime text (Editor), save, and in 2 seconds its done. then clean the browser cache and reload your website.