A child theme automatically inherits the parent theme’s features, styles, and templates. This allows you to make changes to your site using child themes without ever modifying the parent theme. When a new version of the parent theme arrives, you can safely update it as all your modifications are saved in the child theme.
A child theme built on a powerful theme framework allows a great deal of flexibility without writing a lot of code. You can selectively modify only the template files and functions that you need without going through other template files. You can add new functionality and much more. (from http://www.wpbeginner.com/beginners-guide/wordpress-child-theme-pros-cons/)
Install the Child Theme
Navigate to Appearance > Themes and click on Add New or Add New Theme as displayed in the image below.
Click on Upload Theme and Choose File,
Select houzez-child.zip – This is the child theme you to upload and activate
Be sure the child theme is active under Appearance > Themes
Edit the Child Theme
Navigate to Appearance > Editor and select the theme to edit.
To add your custom CSS code select styles.css, add the code in the editor and Update File.