Styling and function replacement in child theme?

Hello,

First, thanks for your simple and clean theme.

I am using a child theme. In my child theme folder, I have only style.css file to modify the styles.

2. Yet, I want to amend the styles in from lib folder, too. How could I do it in my child theme?

1. Another one is also apply like above, too. I want to use some functions which is already have in parent theme; how could I process it within my child theme?

Thank you very much!

Rithy