It is very important to install child theme in case you are going to modify theme files according to your needs. The greatest advantage of the child theme is that you can store the modified theme files in it and they will never be lost after you update the parent theme.
Because child theme doesn’t require updates.
First of all you have to install the parent theme, as described in section 'Theme installation'. Find 'polo-child.zip' file in theme package. Install it the same way as the parent theme over Appearance>Themes>Add new. And activate it.
Child theme fully inherits functionality of the parent theme. In order to edit PHP files, you have to copy them to the child theme preserving the same path structure as in the parent theme. For example, you need to edit file news-page.php that is stored in 'page-templates' folder. You have to create 'page-templates' folder in the child theme and upload modified file in it. So the path must be as follows '.../page-templates/news-page.php'. PLEASE NOTE, this method works only for the root files, and for the files in 'page-templates' and 'post-formats' folders. Other PHP files are built via functions, so in order to edit them, you need to deactivate the original function and add your own in functions.php file of the child theme. You can find more information on overriding parent theme functions in this topic or search for more information in the net. In order to override JS files, you need to dequeue existing script and enqueue your own. More information on this you can find here or here.
You can insert your custom CSS codes directly to the style.css file, that is located in the child theme folder by default or to Theme Options>Advanced Options>Custom CSS Code section. Both ways are correct.