Thanks for writing in.
After installing child theme, https://theme.co/apex/forum/t/setup-how-to-setup-child-themes/57
The best way to edit your child theme style.css is to use FTP and an editor like notepadd or notepad++. Edit your style.css there then reupload it again via FTP. Although it is an easy way however you can easily manage and check your CSS via the editor that you want to use.
For more guide about customization you can check the link below.
In regards to your last question, would you mind sharing us the screenshot that you want to ask.