A child theme is a theme that inherits the functionality and styling of another theme, called the parent theme. Child themes are the recommended way of modifying an existing theme.
A child theme consists of at least one directory (the child theme directory) and two files (style.css and functions.php).
Put below code in child theme style.css
Theme Name: Aviweb Child Theme
Description: This is a child theme for Aviweb
Create functions.php file within child theme folder and write our own custom code.