I am sorry about the confusion. The way the theme is built is that it ignores the custom.css file in the child theme. In fact, I would urge you to ignore custom.css file and put the site specific CSS into child theme style.css since that is the standard recommended practice.
The custom.css is a little inconvenient way to do this since its contents can get erased on updating the theme. We planned to remove the custom.css from the theme but kept the same for backward compatibility reasons since there may be some who have some CSS entered in custom.css file of the parent theme.
Pls move the custom.css file contents to the style.css of the child theme and you should do fine. Thanks